changes

.gitignore

@@ -26,6 +26,10 @@ dist-ssr
 # Tauri build output
 src-tauri/target/
 
+# Google OAuth desktop client (real values, never commit)
+/google-credentials.json
+/client_secret_*.json
+
 # Screenshots & Playwright MCP artifacts
 .playwright-mcp/
 /*.png

CLAUDE.md

@@ -10,7 +10,7 @@
 
 ## Git Commit Rules
 
+  - Do not make branches, commit in main only
   - Use conventional commit format: `feat|fix|refactor|docs|test|chore|ci(scope): message`
   - Never use `git add .` or `git add -A`. Always stage specific files by name.
   - Don't batch multiple unrelated changes into one commit.
-  - Commit early and often. A working 5-line change is better than a pending 200-line change.

google-credentials.example.json

@@ -0,0 +1,11 @@
+{
+  "installed": {
+    "client_id": "YOUR_CLIENT_ID.apps.googleusercontent.com",
+    "project_id": "your-project-id",
+    "auth_uri": "https://accounts.google.com/o/oauth2/auth",
+    "token_uri": "https://oauth2.googleapis.com/token",
+    "auth_provider_x509_cert_url": "https://www.googleapis.com/oauth2/v1/certs",
+    "client_secret": "YOUR_CLIENT_SECRET",
+    "redirect_uris": ["http://127.0.0.1"]
+  }
+}

src-tauri/Cargo.lock

@@ -3089,8 +3089,10 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ff2abc00be7fca6ebc474524697ae276ad847ad0a6b3faa4bcb027e9a4614ad0"
 dependencies = [
  "cfg-if",
+ "js-sys",
  "libc",
  "wasi",
+ "wasm-bindgen",
 ]
 
 [[package]]
@@ -3754,6 +3756,7 @@ dependencies = [
  "tokio",
  "tokio-rustls",
  "tower-service",
+ "webpki-roots",
 ]
 
 [[package]]
@@ -4632,6 +4635,12 @@ dependencies = [
  "hashbrown 0.17.1",
 ]
 
+[[package]]
+name = "lru-slab"
+version = "0.1.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "112b39cec0b298b6c1999fee3e31427f74f676e4cb9879ed1a121b43661a4154"
+
 [[package]]
 name = "lzma-rs"
 version = "0.3.0"
@@ -4709,8 +4718,11 @@ dependencies = [
  "base64 0.22.1",
  "fontdb",
  "harper-core",
+ "rand 0.8.6",
+ "reqwest 0.12.28",
  "serde",
  "serde_json",
+ "sha2",
  "spellbook",
  "tauri",
  "tauri-build",
@@ -4721,6 +4733,7 @@ dependencies = [
  "typst",
  "typst-as-lib",
  "typst-pdf",
+ "url",
  "zip 2.4.2",
 ]
 
@@ -5967,6 +5980,61 @@ dependencies = [
  "memchr",
 ]
 
+[[package]]
+name = "quinn"
+version = "0.11.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0c1a41e437b6bbd489372cd4971de128e85c855f56c57f283d20ff016cf7c0a8"
+dependencies = [
+ "bytes",
+ "cfg_aliases",
+ "pin-project-lite",
+ "quinn-proto",
+ "quinn-udp",
+ "rustc-hash 2.1.2",
+ "rustls",
+ "socket2",
+ "thiserror 2.0.18",
+ "tokio",
+ "tracing",
+ "web-time",
+]
+
+[[package]]
+name = "quinn-proto"
+version = "0.11.15"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "4fcb935c5bec503c2f0e306bdd3e58bb9029dcb14fa8d9ac76e3a5256ac0763e"
+dependencies = [
+ "bytes",
+ "getrandom 0.3.4",
+ "lru-slab",
+ "rand 0.9.4",
+ "ring",
+ "rustc-hash 2.1.2",
+ "rustls",
+ "rustls-pki-types",
+ "slab",
+ "thiserror 2.0.18",
+ "tinyvec",
+ "tracing",
+ "web-time",
+]
+
+[[package]]
+name = "quinn-udp"
+version = "0.5.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "addec6a0dcad8a8d96a771f815f0eaf55f9d1805756410b39f5fa81332574cbd"
+dependencies = [
+ "cfg_aliases",
+ "libc",
+ "once_cell",
+ "socket2",
+ "tracing",
+ "windows-sys 0.60.2",
+]
+
 [[package]]
 name = "quote"
 version = "1.0.45"
@@ -5994,6 +6062,8 @@ version = "0.8.6"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "5ca0ecfa931c29007047d1bc58e623ab12e5590e8c7cc53200d5202b69266d8a"
 dependencies = [
+ "libc",
+ "rand_chacha 0.3.1",
  "rand_core 0.6.4",
  "serde",
 ]
@@ -6034,6 +6104,7 @@ version = "0.6.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ec0be4795e2f6a28069bec0b5ff3e2ac9bafc99e6a9a7dc3547996c5c816922c"
 dependencies = [
+ "getrandom 0.2.17",
  "serde",
 ]
 
@@ -6204,6 +6275,44 @@ version = "1.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "19b30a45b0cd0bcca8037f3d0dc3421eaf95327a17cad11964fb8179b4fc4832"
 
+[[package]]
+name = "reqwest"
+version = "0.12.28"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "eddd3ca559203180a307f12d114c268abf583f59b03cb906fd0b3ff8646c1147"
+dependencies = [
+ "base64 0.22.1",
+ "bytes",
+ "futures-core",
+ "http",
+ "http-body",
+ "http-body-util",
+ "hyper",
+ "hyper-rustls",
+ "hyper-util",
+ "js-sys",
+ "log",
+ "percent-encoding",
+ "pin-project-lite",
+ "quinn",
+ "rustls",
+ "rustls-pki-types",
+ "serde",
+ "serde_json",
+ "serde_urlencoded",
+ "sync_wrapper",
+ "tokio",
+ "tokio-rustls",
+ "tower",
+ "tower-http",
+ "tower-service",
+ "url",
+ "wasm-bindgen",
+ "wasm-bindgen-futures",
+ "web-sys",
+ "webpki-roots",
+]
+
 [[package]]
 name = "reqwest"
 version = "0.13.4"
@@ -6426,6 +6535,7 @@ version = "1.14.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "30a7197ae7eb376e574fe940d068c30fe0462554a3ddbe4eca7838e049c937a9"
 dependencies = [
+ "web-time",
  "zeroize",
 ]
 
@@ -6774,6 +6884,18 @@ dependencies = [
  "serde_core",
 ]
 
+[[package]]
+name = "serde_urlencoded"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3491c14715ca2294c4d6a88f15e84739788c1d030eed8c110436aafdaa2f3fd"
+dependencies = [
+ "form_urlencoded",
+ "itoa",
+ "ryu",
+ "serde",
+]
+
 [[package]]
 name = "serde_with"
 version = "3.21.0"
@@ -7422,7 +7544,7 @@ dependencies = [
  "percent-encoding",
  "plist",
  "raw-window-handle",
- "reqwest",
+ "reqwest 0.13.4",
  "serde",
  "serde_json",
  "serde_repr",
@@ -7611,7 +7733,7 @@ dependencies = [
  "minisign-verify",
  "osakit",
  "percent-encoding",
- "reqwest",
+ "reqwest 0.13.4",
  "rustls",
  "semver",
  "serde",
@@ -9150,6 +9272,15 @@ dependencies = [
  "rustls-pki-types",
 ]
 
+[[package]]
+name = "webpki-roots"
+version = "1.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bf85cb06032201fa7c6f829d7db5a7e5aa45bcc0655327713065f6f0576731bf"
+dependencies = [
+ "rustls-pki-types",
+]
+
 [[package]]
 name = "webview2-com"
 version = "0.38.2"

src-tauri/Cargo.toml

@@ -36,6 +36,10 @@ spellbook = "0.4"
 # bump could silently invalidate the patch ("unused") and pull the broken CUDA subtree back in,
 # so bump deliberately and re-audit the stubs. (see [patch.crates-io])
 harper-core = { version = "=2.5.0", features = ["concurrent"] }
+reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "json"] }
+sha2 = "0.10"
+rand = "0.8"
+url = "2"
 
 # harper-core transitively declares an optional, disabled `burn-cuda` (CUDA backend).
 # Cargo still version-resolves that dead subtree, which currently fails (broken tracel-llvm

src-tauri/src/gdrive.rs

@@ -0,0 +1,806 @@
+use base64::Engine;
+use base64::engine::general_purpose::URL_SAFE_NO_PAD;
+use rand::RngCore;
+use serde::de::DeserializeOwned;
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+use std::collections::HashMap;
+use std::fs;
+use std::io::{Read, Write};
+use std::net::{TcpListener, TcpStream};
+use std::path::PathBuf;
+use std::sync::{LazyLock, Mutex};
+use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
+use tauri::{Emitter, Manager};
+
+const FOLDER_NAME: &str = "margin";
+const DICTIONARY_NAME: &str = "custom-dictionary.txt";
+const SCOPES: &str = "openid email https://www.googleapis.com/auth/drive.file";
+const AUTH_TIMEOUT_SECS: u64 = 120;
+
+const CREDENTIALS_JSON: &str =
+    include_str!(concat!(env!("CARGO_MANIFEST_DIR"), "/../google-credentials.json"));
+
+static HTTP: LazyLock<reqwest::Client> = LazyLock::new(reqwest::Client::new);
+
+#[derive(Deserialize)]
+struct CredentialsFile {
+    installed: Credentials,
+}
+
+#[derive(Deserialize)]
+struct Credentials {
+    client_id: String,
+    client_secret: String,
+    auth_uri: String,
+    token_uri: String,
+}
+
+fn load_credentials() -> Result<Credentials, String> {
+    let runtime_path = concat!(env!("CARGO_MANIFEST_DIR"), "/../google-credentials.json");
+    let raw = fs::read_to_string(runtime_path).unwrap_or_else(|_| CREDENTIALS_JSON.to_string());
+    let parsed: CredentialsFile = serde_json::from_str(&raw)
+        .map_err(|e| format!("invalid google-credentials.json: {e}"))?;
+    let creds = parsed.installed;
+    if creds.client_id.starts_with("YOUR_CLIENT_ID") || creds.client_secret.starts_with("YOUR_CLIENT_SECRET") {
+        return Err("Google Drive is not set up yet. Add a real OAuth desktop client to google-credentials.json.".to_string());
+    }
+    Ok(creds)
+}
+
+#[derive(Default)]
+pub struct GDriveState(pub Mutex<Session>);
+
+#[derive(Default)]
+pub struct Session {
+    refresh_token: Option<String>,
+    access_token: Option<String>,
+    access_expiry: u64,
+    email: Option<String>,
+    folder_id: Option<String>,
+}
+
+#[derive(Serialize, Deserialize, Default)]
+struct BackupState {
+    #[serde(default)]
+    refresh_token: Option<String>,
+    email: Option<String>,
+    folder_id: Option<String>,
+    last_backup: Option<u64>,
+    #[serde(default)]
+    files: HashMap<String, FileRecord>,
+}
+
+#[derive(Serialize, Deserialize, Clone)]
+struct FileRecord {
+    hash: String,
+    drive_id: String,
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct Status {
+    connected: bool,
+    email: Option<String>,
+    last_backup: Option<u64>,
+    pending: bool,
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct RestoreResult {
+    restored: usize,
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct BackupOutcome {
+    uploaded: usize,
+    #[serde(flatten)]
+    status: Status,
+}
+
+#[derive(Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct RemoteBackup {
+    name: String,
+}
+
+#[derive(Clone, Serialize)]
+#[serde(rename_all = "camelCase")]
+struct AuthEvent {
+    ok: bool,
+    error: Option<String>,
+}
+
+fn now() -> u64 {
+    SystemTime::now().duration_since(UNIX_EPOCH).unwrap_or_default().as_secs()
+}
+
+fn dictionary_path(app: &tauri::AppHandle) -> Result<PathBuf, String> {
+    Ok(crate::library::app_data_dir(app)?.join(DICTIONARY_NAME))
+}
+
+fn state_path(app: &tauri::AppHandle) -> Result<PathBuf, String> {
+    Ok(crate::library::app_data_dir(app)?.join("backup.json"))
+}
+
+fn load_state(app: &tauri::AppHandle) -> BackupState {
+    let path = match state_path(app) {
+        Ok(p) => p,
+        Err(_) => return BackupState::default(),
+    };
+    match fs::read_to_string(&path) {
+        Ok(contents) => serde_json::from_str(&contents).unwrap_or_default(),
+        Err(_) => BackupState::default(),
+    }
+}
+
+fn save_state(app: &tauri::AppHandle, state: &BackupState) -> Result<(), String> {
+    let path = state_path(app)?;
+    let contents = serde_json::to_string_pretty(state).map_err(|e| e.to_string())?;
+    crate::project::atomic_write(&path, contents.as_bytes(), false)
+}
+
+fn random_b64(bytes: usize) -> String {
+    let mut buf = vec![0u8; bytes];
+    rand::thread_rng().fill_bytes(&mut buf);
+    URL_SAFE_NO_PAD.encode(buf)
+}
+
+fn pkce_challenge(verifier: &str) -> String {
+    let mut hasher = Sha256::new();
+    hasher.update(verifier.as_bytes());
+    URL_SAFE_NO_PAD.encode(hasher.finalize())
+}
+
+fn hash_bytes(bytes: &[u8]) -> String {
+    let mut hasher = Sha256::new();
+    hasher.update(bytes);
+    format!("{:x}", hasher.finalize())
+}
+
+fn urlencode(s: &str) -> String {
+    url::form_urlencoded::byte_serialize(s.as_bytes()).collect()
+}
+
+fn drive_escape(s: &str) -> String {
+    s.replace('\\', "\\\\").replace('\'', "\\'")
+}
+
+fn safe_book_name(name: &str) -> bool {
+    match name.strip_suffix(".margin") {
+        Some(stem) => !stem.is_empty() && stem.chars().all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_'),
+        None => false,
+    }
+}
+
+fn write_http_message(stream: &mut TcpStream, message: &str) {
+    let body = format!(
+        "<!doctype html><html><head><meta charset=\"utf-8\"><title>margin</title></head>\
+         <body style=\"font-family:system-ui,sans-serif;text-align:center;padding-top:80px;color:#222\">\
+         <h2>{message}</h2></body></html>"
+    );
+    let response = format!(
+        "HTTP/1.1 200 OK\r\nContent-Type: text/html; charset=utf-8\r\nContent-Length: {}\r\nConnection: close\r\n\r\n{}",
+        body.len(),
+        body
+    );
+    let _ = stream.write_all(response.as_bytes());
+    let _ = stream.flush();
+}
+
+fn await_code(listener: TcpListener, expected_state: &str, deadline: Instant) -> Result<String, String> {
+    listener.set_nonblocking(true).map_err(|e| e.to_string())?;
+    loop {
+        if Instant::now() > deadline {
+            return Err("Timed out waiting for Google authorization.".to_string());
+        }
+        match listener.accept() {
+            Ok((mut stream, _)) => {
+                stream.set_nonblocking(false).ok();
+                stream.set_read_timeout(Some(Duration::from_secs(5))).ok();
+                let mut buf = [0u8; 8192];
+                let n = stream.read(&mut buf).unwrap_or(0);
+                let request = String::from_utf8_lossy(&buf[..n]);
+                let path = request.lines().next().and_then(|line| line.split_whitespace().nth(1)).unwrap_or("");
+                if path == "/favicon.ico" {
+                    write_http_message(&mut stream, "margin");
+                    continue;
+                }
+                let full = format!("http://127.0.0.1{path}");
+                let parsed = match url::Url::parse(&full) {
+                    Ok(p) => p,
+                    Err(_) => {
+                        write_http_message(&mut stream, "Waiting for Google…");
+                        continue;
+                    }
+                };
+                let mut code = None;
+                let mut state = None;
+                let mut error = None;
+                for (key, value) in parsed.query_pairs() {
+                    match key.as_ref() {
+                        "code" => code = Some(value.into_owned()),
+                        "state" => state = Some(value.into_owned()),
+                        "error" => error = Some(value.into_owned()),
+                        _ => {}
+                    }
+                }
+                if let Some(error) = error {
+                    write_http_message(&mut stream, "Authorization was cancelled. You can close this tab.");
+                    return Err(format!("Google authorization failed: {error}"));
+                }
+                match (code, state) {
+                    (Some(code), Some(state)) if state == expected_state => {
+                        write_http_message(&mut stream, "Connected to margin. You can close this tab.");
+                        return Ok(code);
+                    }
+                    (Some(_), _) => {
+                        write_http_message(&mut stream, "Could not verify the request. You can close this tab.");
+                        return Err("State mismatch during Google authorization.".to_string());
+                    }
+                    _ => {
+                        write_http_message(&mut stream, "Waiting for Google…");
+                    }
+                }
+            }
+            Err(ref e) if e.kind() == std::io::ErrorKind::WouldBlock => {
+                std::thread::sleep(Duration::from_millis(150));
+            }
+            Err(e) => return Err(e.to_string()),
+        }
+    }
+}
+
+#[derive(Deserialize)]
+struct TokenResponse {
+    access_token: String,
+    #[serde(default)]
+    refresh_token: Option<String>,
+    #[serde(default)]
+    expires_in: u64,
+}
+
+#[derive(Deserialize)]
+struct UserInfo {
+    #[serde(default)]
+    email: Option<String>,
+}
+
+#[derive(Deserialize)]
+struct DriveFile {
+    id: String,
+    #[serde(default)]
+    name: String,
+}
+
+#[derive(Deserialize)]
+struct FileList {
+    #[serde(default)]
+    files: Vec<DriveFile>,
+    #[serde(default, rename = "nextPageToken")]
+    next_page_token: Option<String>,
+}
+
+async fn read_json<T: DeserializeOwned>(resp: reqwest::Response, context: &str) -> Result<T, String> {
+    let status = resp.status();
+    let text = resp.text().await.map_err(|e| e.to_string())?;
+    if !status.is_success() {
+        return Err(format!("{context} failed ({status}): {text}"));
+    }
+    serde_json::from_str(&text).map_err(|e| format!("{context}: could not parse response: {e}"))
+}
+
+async fn exchange_code(creds: &Credentials, code: &str, redirect: &str, verifier: &str) -> Result<TokenResponse, String> {
+    let resp = HTTP
+        .post(&creds.token_uri)
+        .form(&[
+            ("client_id", creds.client_id.as_str()),
+            ("client_secret", creds.client_secret.as_str()),
+            ("code", code),
+            ("code_verifier", verifier),
+            ("grant_type", "authorization_code"),
+            ("redirect_uri", redirect),
+        ])
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    read_json(resp, "Google token exchange").await
+}
+
+async fn refresh_access_token(creds: &Credentials, refresh_token: &str) -> Result<TokenResponse, String> {
+    let resp = HTTP
+        .post(&creds.token_uri)
+        .form(&[
+            ("client_id", creds.client_id.as_str()),
+            ("client_secret", creds.client_secret.as_str()),
+            ("refresh_token", refresh_token),
+            ("grant_type", "refresh_token"),
+        ])
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    read_json(resp, "Google token refresh").await
+}
+
+async fn fetch_email(access_token: &str) -> Result<String, String> {
+    let resp = HTTP
+        .get("https://www.googleapis.com/oauth2/v2/userinfo")
+        .bearer_auth(access_token)
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    let info: UserInfo = read_json(resp, "Google account lookup").await?;
+    Ok(info.email.unwrap_or_default())
+}
+
+async fn ensure_folder(access_token: &str) -> Result<String, String> {
+    let query = format!(
+        "name = '{}' and mimeType = 'application/vnd.google-apps.folder' and trashed = false",
+        drive_escape(FOLDER_NAME)
+    );
+    let resp = HTTP
+        .get("https://www.googleapis.com/drive/v3/files")
+        .bearer_auth(access_token)
+        .query(&[
+            ("q", query.as_str()),
+            ("fields", "files(id,name)"),
+            ("spaces", "drive"),
+            ("pageSize", "1"),
+        ])
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    let list: FileList = read_json(resp, "Drive folder lookup").await?;
+    if let Some(folder) = list.files.into_iter().next() {
+        return Ok(folder.id);
+    }
+    let body = serde_json::json!({
+        "name": FOLDER_NAME,
+        "mimeType": "application/vnd.google-apps.folder",
+    });
+    let resp = HTTP
+        .post("https://www.googleapis.com/drive/v3/files")
+        .bearer_auth(access_token)
+        .query(&[("fields", "id")])
+        .json(&body)
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    let created: DriveFile = read_json(resp, "Drive folder creation").await?;
+    Ok(created.id)
+}
+
+async fn find_file(access_token: &str, folder_id: &str, name: &str) -> Result<Option<DriveFile>, String> {
+    let query = format!(
+        "'{}' in parents and name = '{}' and trashed = false",
+        drive_escape(folder_id),
+        drive_escape(name)
+    );
+    let resp = HTTP
+        .get("https://www.googleapis.com/drive/v3/files")
+        .bearer_auth(access_token)
+        .query(&[
+            ("q", query.as_str()),
+            ("fields", "files(id,name)"),
+            ("spaces", "drive"),
+            ("pageSize", "1"),
+        ])
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    let list: FileList = read_json(resp, "Drive file lookup").await?;
+    Ok(list.files.into_iter().next())
+}
+
+async fn list_in_folder(access_token: &str, folder_id: &str) -> Result<Vec<DriveFile>, String> {
+    let query = format!("'{}' in parents and trashed = false", drive_escape(folder_id));
+    let mut files = Vec::new();
+    let mut page_token: Option<String> = None;
+    loop {
+        let mut request = HTTP
+            .get("https://www.googleapis.com/drive/v3/files")
+            .bearer_auth(access_token)
+            .query(&[
+                ("q", query.as_str()),
+                ("fields", "nextPageToken,files(id,name)"),
+                ("spaces", "drive"),
+                ("pageSize", "100"),
+            ]);
+        if let Some(token) = &page_token {
+            request = request.query(&[("pageToken", token.as_str())]);
+        }
+        let resp = request.send().await.map_err(|e| e.to_string())?;
+        let FileList { files: page, next_page_token } = read_json(resp, "Drive file list").await?;
+        files.extend(page);
+        match next_page_token {
+            Some(token) => page_token = Some(token),
+            None => break,
+        }
+    }
+    Ok(files)
+}
+
+async fn upload_file(
+    access_token: &str,
+    folder_id: &str,
+    name: &str,
+    bytes: &[u8],
+    existing_id: Option<String>,
+) -> Result<DriveFile, String> {
+    let boundary = "margin7f3e2a1b9c8d";
+    let metadata = match &existing_id {
+        Some(_) => serde_json::json!({ "name": name }),
+        None => serde_json::json!({ "name": name, "parents": [folder_id] }),
+    };
+    let mut body: Vec<u8> = Vec::new();
+    body.extend_from_slice(format!("--{boundary}\r\nContent-Type: application/json; charset=UTF-8\r\n\r\n").as_bytes());
+    body.extend_from_slice(serde_json::to_string(&metadata).unwrap_or_default().as_bytes());
+    body.extend_from_slice(format!("\r\n--{boundary}\r\nContent-Type: application/octet-stream\r\n\r\n").as_bytes());
+    body.extend_from_slice(bytes);
+    body.extend_from_slice(format!("\r\n--{boundary}--\r\n").as_bytes());
+
+    let request = match &existing_id {
+        Some(id) => HTTP.patch(format!("https://www.googleapis.com/upload/drive/v3/files/{id}")),
+        None => HTTP.post("https://www.googleapis.com/upload/drive/v3/files"),
+    };
+    let resp = request
+        .bearer_auth(access_token)
+        .query(&[("uploadType", "multipart"), ("fields", "id")])
+        .header(reqwest::header::CONTENT_TYPE, format!("multipart/related; boundary={boundary}"))
+        .body(body)
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    read_json(resp, &format!("Drive upload of {name}")).await
+}
+
+async fn download_file(access_token: &str, id: &str) -> Result<Vec<u8>, String> {
+    let resp = HTTP
+        .get(format!("https://www.googleapis.com/drive/v3/files/{id}"))
+        .bearer_auth(access_token)
+        .query(&[("alt", "media")])
+        .send()
+        .await
+        .map_err(|e| e.to_string())?;
+    let status = resp.status();
+    if !status.is_success() {
+        let text = resp.text().await.unwrap_or_default();
+        return Err(format!("Drive download failed ({status}): {text}"));
+    }
+    Ok(resp.bytes().await.map_err(|e| e.to_string())?.to_vec())
+}
+
+async fn valid_access_token(app: &tauri::AppHandle, state: &GDriveState) -> Result<String, String> {
+    let (access, expiry, refresh) = {
+        let session = state.0.lock().unwrap();
+        (session.access_token.clone(), session.access_expiry, session.refresh_token.clone())
+    };
+    if let Some(token) = access {
+        if now() < expiry {
+            return Ok(token);
+        }
+    }
+    let refresh = refresh.ok_or("Not connected to Google Drive.")?;
+    let creds = load_credentials()?;
+    let tokens = refresh_access_token(&creds, &refresh).await?;
+    let access_token = tokens.access_token.clone();
+    {
+        let mut session = state.0.lock().unwrap();
+        session.access_token = Some(tokens.access_token);
+        session.access_expiry = now() + tokens.expires_in.saturating_sub(60);
+        if let Some(rotated) = &tokens.refresh_token {
+            session.refresh_token = Some(rotated.clone());
+        }
+    }
+    if let Some(rotated) = &tokens.refresh_token {
+        let mut stored = load_state(app);
+        stored.refresh_token = Some(rotated.clone());
+        let _ = save_state(app, &stored);
+    }
+    Ok(access_token)
+}
+
+async fn ensure_folder_id(app: &tauri::AppHandle, state: &GDriveState, access_token: &str) -> Result<String, String> {
+    {
+        let session = state.0.lock().unwrap();
+        if let Some(folder) = &session.folder_id {
+            return Ok(folder.clone());
+        }
+    }
+    let folder_id = ensure_folder(access_token).await?;
+    {
+        let mut session = state.0.lock().unwrap();
+        session.folder_id = Some(folder_id.clone());
+    }
+    let mut stored = load_state(app);
+    stored.folder_id = Some(folder_id.clone());
+    save_state(app, &stored)?;
+    Ok(folder_id)
+}
+
+fn collect_local_files(app: &tauri::AppHandle) -> Result<Vec<(String, Vec<u8>)>, String> {
+    let mut files = Vec::new();
+    let library = crate::library::library_dir(app)?;
+    for entry in fs::read_dir(&library).map_err(|e| e.to_string())? {
+        let path = match entry {
+            Ok(entry) => entry.path(),
+            Err(_) => continue,
+        };
+        if path.extension().and_then(|e| e.to_str()) != Some("margin") {
+            continue;
+        }
+        let name = match path.file_name().and_then(|n| n.to_str()) {
+            Some(name) if safe_book_name(name) => name.to_string(),
+            _ => continue,
+        };
+        let bytes = fs::read(&path).map_err(|e| e.to_string())?;
+        files.push((name, bytes));
+    }
+    let dictionary = dictionary_path(app)?;
+    if dictionary.exists() {
+        let bytes = fs::read(&dictionary).map_err(|e| e.to_string())?;
+        files.push((DICTIONARY_NAME.to_string(), bytes));
+    }
+    Ok(files)
+}
+
+fn file_pending(path: &std::path::Path, name: &str, stored: &BackupState, last_backup: u64) -> bool {
+    let record = match stored.files.get(name) {
+        Some(record) => record,
+        None => return true,
+    };
+    let touched = fs::metadata(path)
+        .and_then(|meta| meta.modified())
+        .ok()
+        .and_then(|time| time.duration_since(UNIX_EPOCH).ok())
+        .map(|since| since.as_secs() > last_backup)
+        .unwrap_or(true);
+    if !touched {
+        return false;
+    }
+    match fs::read(path) {
+        Ok(bytes) => record.hash != hash_bytes(&bytes),
+        Err(_) => false,
+    }
+}
+
+fn compute_pending(app: &tauri::AppHandle, stored: &BackupState) -> bool {
+    let last_backup = stored.last_backup.unwrap_or(0);
+    let library = match crate::library::library_dir(app) {
+        Ok(dir) => dir,
+        Err(_) => return false,
+    };
+    if let Ok(entries) = fs::read_dir(&library) {
+        for entry in entries.flatten() {
+            let path = entry.path();
+            if path.extension().and_then(|e| e.to_str()) != Some("margin") {
+                continue;
+            }
+            if let Some(name) = path.file_name().and_then(|n| n.to_str()) {
+                if safe_book_name(name) && file_pending(&path, name, stored, last_backup) {
+                    return true;
+                }
+            }
+        }
+    }
+    match dictionary_path(app) {
+        Ok(dict) if dict.exists() => file_pending(&dict, DICTIONARY_NAME, stored, last_backup),
+        _ => false,
+    }
+}
+
+fn status_inner(app: &tauri::AppHandle, state: &GDriveState) -> Status {
+    let (connected, email) = {
+        let session = state.0.lock().unwrap();
+        (session.refresh_token.is_some(), session.email.clone())
+    };
+    let stored = load_state(app);
+    let pending = connected && compute_pending(app, &stored);
+    Status {
+        connected,
+        email: email.or(stored.email),
+        last_backup: stored.last_backup,
+        pending,
+    }
+}
+
+pub fn init_session(app: &tauri::AppHandle) {
+    let state = app.state::<GDriveState>();
+    let stored = load_state(app);
+    let mut session = state.0.lock().unwrap();
+    session.refresh_token = stored.refresh_token;
+    session.email = stored.email;
+    session.folder_id = stored.folder_id;
+}
+
+async fn complete_auth(
+    app: &tauri::AppHandle,
+    listener: TcpListener,
+    csrf: String,
+    verifier: String,
+    redirect: String,
+    creds: Credentials,
+) -> Result<(), String> {
+    let code = tauri::async_runtime::spawn_blocking(move || {
+        let deadline = Instant::now() + Duration::from_secs(AUTH_TIMEOUT_SECS);
+        await_code(listener, &csrf, deadline)
+    })
+    .await
+    .map_err(|e| e.to_string())??;
+
+    let tokens = exchange_code(&creds, &code, &redirect, &verifier).await?;
+    let email = fetch_email(&tokens.access_token).await?;
+    let folder_id = ensure_folder(&tokens.access_token).await?;
+
+    {
+        let state = app.state::<GDriveState>();
+        let mut session = state.0.lock().unwrap();
+        if let Some(refresh) = &tokens.refresh_token {
+            session.refresh_token = Some(refresh.clone());
+        }
+        session.access_token = Some(tokens.access_token.clone());
+        session.access_expiry = now() + tokens.expires_in.saturating_sub(60);
+        session.email = Some(email.clone());
+        session.folder_id = Some(folder_id.clone());
+    }
+
+    let mut stored = load_state(app);
+    if let Some(refresh) = &tokens.refresh_token {
+        stored.refresh_token = Some(refresh.clone());
+    }
+    stored.email = Some(email);
+    stored.folder_id = Some(folder_id);
+    save_state(app, &stored)?;
+    Ok(())
+}
+
+#[tauri::command]
+pub async fn gdrive_connect(app: tauri::AppHandle) -> Result<String, String> {
+    let creds = load_credentials()?;
+    let verifier = random_b64(64);
+    let challenge = pkce_challenge(&verifier);
+    let csrf = random_b64(24);
+
+    let listener = TcpListener::bind("127.0.0.1:0").map_err(|e| e.to_string())?;
+    let port = listener.local_addr().map_err(|e| e.to_string())?.port();
+    let redirect = format!("http://127.0.0.1:{port}");
+
+    let auth_url = format!(
+        "{}?client_id={}&redirect_uri={}&response_type=code&scope={}&code_challenge={}&code_challenge_method=S256&state={}&access_type=offline&prompt=consent",
+        creds.auth_uri,
+        urlencode(&creds.client_id),
+        urlencode(&redirect),
+        urlencode(SCOPES),
+        challenge,
+        urlencode(&csrf),
+    );
+
+    use tauri_plugin_opener::OpenerExt;
+    let _ = app.opener().open_url(auth_url.clone(), None::<&str>);
+
+    let app_bg = app.clone();
+    tauri::async_runtime::spawn(async move {
+        let event = match complete_auth(&app_bg, listener, csrf, verifier, redirect, creds).await {
+            Ok(()) => AuthEvent { ok: true, error: None },
+            Err(error) => AuthEvent { ok: false, error: Some(error) },
+        };
+        let _ = app_bg.emit("gdrive-auth", event);
+    });
+
+    Ok(auth_url)
+}
+
+#[tauri::command]
+pub async fn gdrive_disconnect(app: tauri::AppHandle, state: tauri::State<'_, GDriveState>) -> Result<Status, String> {
+    let refresh = {
+        let session = state.0.lock().unwrap();
+        session.refresh_token.clone()
+    };
+    if let Some(refresh) = refresh {
+        let _ = HTTP
+            .post("https://oauth2.googleapis.com/revoke")
+            .form(&[("token", refresh.as_str())])
+            .send()
+            .await;
+    }
+    {
+        let mut session = state.0.lock().unwrap();
+        session.refresh_token = None;
+        session.access_token = None;
+        session.access_expiry = 0;
+        session.email = None;
+    }
+    let mut stored = load_state(&app);
+    stored.refresh_token = None;
+    stored.email = None;
+    save_state(&app, &stored)?;
+    Ok(status_inner(&app, &state))
+}
+
+#[tauri::command]
+pub async fn gdrive_status(app: tauri::AppHandle, state: tauri::State<'_, GDriveState>) -> Result<Status, String> {
+    Ok(status_inner(&app, &state))
+}
+
+#[tauri::command]
+pub async fn gdrive_backup(app: tauri::AppHandle, state: tauri::State<'_, GDriveState>) -> Result<BackupOutcome, String> {
+    let access_token = valid_access_token(&app, &state).await?;
+    let folder_id = ensure_folder_id(&app, &state, &access_token).await?;
+    let files = collect_local_files(&app)?;
+    let mut stored = load_state(&app);
+    let mut uploaded = 0;
+    for (name, bytes) in &files {
+        let hash = hash_bytes(bytes);
+        let existing = stored.files.get(name).cloned();
+        if let Some(record) = &existing {
+            if record.hash == hash {
+                continue;
+            }
+        }
+        let drive_id = match &existing {
+            Some(record) => Some(record.drive_id.clone()),
+            None => find_file(&access_token, &folder_id, name).await?.map(|file| file.id),
+        };
+        let result = upload_file(&access_token, &folder_id, name, bytes, drive_id).await?;
+        stored.files.insert(name.clone(), FileRecord { hash, drive_id: result.id });
+        uploaded += 1;
+    }
+    if uploaded > 0 {
+        stored.last_backup = Some(now());
+        save_state(&app, &stored)?;
+    }
+    Ok(BackupOutcome {
+        uploaded,
+        status: Status {
+            connected: true,
+            email: stored.email.clone(),
+            last_backup: stored.last_backup,
+            pending: false,
+        },
+    })
+}
+
+#[tauri::command]
+pub async fn gdrive_restore(app: tauri::AppHandle, state: tauri::State<'_, GDriveState>) -> Result<RestoreResult, String> {
+    let access_token = valid_access_token(&app, &state).await?;
+    let folder_id = ensure_folder_id(&app, &state, &access_token).await?;
+    let remote = list_in_folder(&access_token, &folder_id).await?;
+    let mut stored = load_state(&app);
+    let mut restored = 0;
+    for file in &remote {
+        let is_dictionary = file.name == DICTIONARY_NAME;
+        let is_book = safe_book_name(&file.name);
+        if !is_dictionary && !is_book {
+            continue;
+        }
+        let bytes = download_file(&access_token, &file.id).await?;
+        let destination = if is_dictionary {
+            dictionary_path(&app)?
+        } else {
+            crate::library::library_dir(&app)?.join(&file.name)
+        };
+        crate::project::atomic_write(&destination, &bytes, false)?;
+        stored.files.insert(
+            file.name.clone(),
+            FileRecord { hash: hash_bytes(&bytes), drive_id: file.id.clone() },
+        );
+        restored += 1;
+    }
+    stored.last_backup = Some(now());
+    save_state(&app, &stored)?;
+    Ok(RestoreResult { restored })
+}
+
+#[tauri::command]
+pub async fn gdrive_list_backups(app: tauri::AppHandle, state: tauri::State<'_, GDriveState>) -> Result<Vec<RemoteBackup>, String> {
+    let access_token = valid_access_token(&app, &state).await?;
+    let folder_id = ensure_folder_id(&app, &state, &access_token).await?;
+    let remote = list_in_folder(&access_token, &folder_id).await?;
+    Ok(remote
+        .into_iter()
+        .filter(|file| safe_book_name(&file.name))
+        .map(|file| RemoteBackup { name: file.name })
+        .collect())
+}

src-tauri/src/lib.rs

@@ -1,5 +1,6 @@
 mod epub;
 mod fonts;
+mod gdrive;
 mod library;
 mod pdf;
 mod project;
@@ -72,6 +73,11 @@ pub fn run() {
 
     builder
         .manage(proofing::new_state())
+        .manage(gdrive::GDriveState::default())
+        .setup(|app| {
+            gdrive::init_session(app.handle());
+            Ok(())
+        })
         .menu(|handle| build_menu(handle))
         .on_menu_event(|app, event| {
             if matches!(
@@ -94,7 +100,13 @@ pub fn run() {
             library::save_book,
             library::delete_book,
             proofing::proof_text,
-            proofing::remember_word
+            proofing::remember_word,
+            gdrive::gdrive_connect,
+            gdrive::gdrive_disconnect,
+            gdrive::gdrive_status,
+            gdrive::gdrive_backup,
+            gdrive::gdrive_restore,
+            gdrive::gdrive_list_backups
         ])
         .run(context)
         .expect("error while running margin");

src-tauri/src/library.rs

@@ -19,12 +19,14 @@ fn corrupt_summary(stem: &str) -> BookSummary {
     }
 }
 
-fn library_dir(app: &tauri::AppHandle) -> Result<PathBuf, String> {
+pub(crate) fn app_data_dir(app: &tauri::AppHandle) -> Result<PathBuf, String> {
-    let dir = app
+    let dir = app.path().app_data_dir().map_err(|e| e.to_string())?;
-        .path()
+    fs::create_dir_all(&dir).map_err(|e| e.to_string())?;
-        .app_data_dir()
+    Ok(dir)
-        .map_err(|e| e.to_string())?
+}
-        .join("library");
+
+pub(crate) fn library_dir(app: &tauri::AppHandle) -> Result<PathBuf, String> {
+    let dir = app_data_dir(app)?.join("library");
     fs::create_dir_all(&dir).map_err(|e| e.to_string())?;
     Ok(dir)
 }

src/App.tsx

@@ -3,7 +3,9 @@ import { listen } from "@tauri-apps/api/event";
 import { getCurrentWindow } from "@tauri-apps/api/window";
 import { Library } from "./components/Library";
 import { EditorView } from "./components/EditorView";
+import { BackupSettings } from "./components/BackupSettings";
 import { useBook } from "./store/useBook";
+import { useBackup } from "./store/useBackup";
 import { isDesktop } from "./ipc";
 import { createAndOpenBook, saveBook } from "./library";
 import { runExport } from "./export/run";
@@ -37,9 +39,16 @@ function App() {
     const win = getCurrentWindow();
     const unlisten = win.onCloseRequested(async (event) => {
       const { book, dirty } = useBook.getState();
-      if (!book || !dirty) return;
+      const { connected } = useBackup.getState();
+      if ((!book || !dirty) && !connected) return;
       event.preventDefault();
-      await saveBook(book).catch(() => {});
+      if (book && dirty) await saveBook(book).catch(() => {});
+      if (connected) {
+        await Promise.race([
+          useBackup.getState().backup(true),
+          new Promise((resolve) => setTimeout(resolve, 8000)),
+        ]);
+      }
       win.destroy();
     });
     return () => {
@@ -47,7 +56,34 @@ function App() {
     };
   }, []);
 
-  return book ? <EditorView /> : <Library onOpen={openBook} />;
+  useEffect(() => {
+    if (!isDesktop) return;
+    const unlisten = listen<{ ok: boolean; error: string | null }>("gdrive-auth", (event) => {
+      useBackup.getState().handleAuthEvent(event.payload.ok, event.payload.error);
+    });
+    return () => {
+      unlisten.then((stop) => stop());
+    };
+  }, []);
+
+  useEffect(() => {
+    if (!isDesktop) return;
+    const tick = async () => {
+      await useBackup.getState().refresh();
+      const state = useBackup.getState();
+      if (state.connected && state.pending) state.backup(true);
+    };
+    tick();
+    const id = setInterval(tick, 15 * 60 * 1000);
+    return () => clearInterval(id);
+  }, []);
+
+  return (
+    <>
+      {book ? <EditorView /> : <Library onOpen={openBook} />}
+      {isDesktop && <BackupSettings />}
+    </>
+  );
 }
 
 export default App;

src/backup.ts

@@ -0,0 +1,63 @@
+import { invoke } from "@tauri-apps/api/core";
+import { isDesktop } from "./ipc";
+
+export interface BackupStatus {
+  connected: boolean;
+  email: string | null;
+  lastBackup: number | null;
+  pending: boolean;
+}
+
+export interface RestoreResult {
+  restored: number;
+}
+
+export interface BackupOutcome extends BackupStatus {
+  uploaded: number;
+}
+
+export interface RemoteBackup {
+  name: string;
+}
+
+const OFFLINE: BackupStatus = { connected: false, email: null, lastBackup: null, pending: false };
+
+export async function gdriveStatus(): Promise<BackupStatus> {
+  if (!isDesktop) return OFFLINE;
+  return invoke<BackupStatus>("gdrive_status");
+}
+
+export async function gdriveConnect(): Promise<string> {
+  return invoke<string>("gdrive_connect");
+}
+
+export async function gdriveDisconnect(): Promise<BackupStatus> {
+  return invoke<BackupStatus>("gdrive_disconnect");
+}
+
+export async function gdriveBackup(): Promise<BackupOutcome> {
+  return invoke<BackupOutcome>("gdrive_backup");
+}
+
+export async function gdriveRestore(): Promise<RestoreResult> {
+  return invoke<RestoreResult>("gdrive_restore");
+}
+
+export async function gdriveListBackups(): Promise<RemoteBackup[]> {
+  if (!isDesktop) return [];
+  return invoke<RemoteBackup[]>("gdrive_list_backups");
+}
+
+export function formatBackupTime(ts: number | null): string {
+  if (!ts) return "Not backed up yet";
+  const diff = Date.now() - ts * 1000;
+  const min = Math.floor(diff / 60000);
+  if (min < 1) return "Backed up just now";
+  if (min < 60) return `Backed up ${min}m ago`;
+  const hr = Math.floor(min / 60);
+  if (hr < 24) return `Backed up ${hr}h ago`;
+  const day = Math.floor(hr / 24);
+  if (day === 1) return "Backed up yesterday";
+  if (day < 7) return `Backed up ${day}d ago`;
+  return `Backed up ${new Date(ts * 1000).toLocaleDateString()}`;
+}

src/components/BackupButton.tsx

@@ -0,0 +1,48 @@
+import { Icon } from "./Icon";
+import { useBackup } from "../store/useBackup";
+import { formatBackupTime } from "../backup";
+
+export function BackupButton() {
+  const connected = useBackup((s) => s.connected);
+  const pending = useBackup((s) => s.pending);
+  const phase = useBackup((s) => s.phase);
+  const lastBackup = useBackup((s) => s.lastBackup);
+  const openSettings = useBackup((s) => s.openSettings);
+
+  const busy = phase === "working" || phase === "connecting";
+  const error = phase === "error";
+
+  const state = busy ? "working" : error ? "error" : !connected ? "off" : pending ? "pending" : "synced";
+
+  const title =
+    phase === "connecting"
+      ? "Waiting for Google authorization…"
+      : state === "working"
+        ? "Backing up…"
+        : state === "error"
+          ? "Backup needs attention"
+          : state === "off"
+            ? "Connect Google Drive"
+            : state === "pending"
+              ? "Changes not backed up"
+              : formatBackupTime(lastBackup);
+
+  const onClick = () => {
+    openSettings();
+  };
+
+  return (
+    <button className="icon-btn backup-btn" data-state={state} onClick={onClick} title={title}>
+      {busy ? (
+        <Icon size={18}>
+          <path d="M21 12a9 9 0 1 1-2.6-6.4M21 4v5h-5" />
+        </Icon>
+      ) : (
+        <Icon size={18}>
+          <path d="M7 18a4 4 0 0 1 0-8 5 5 0 0 1 9.6-1.3A3.5 3.5 0 0 1 18 18H7z" />
+          {connected ? <path d="M12 16.5v-5M9.7 12.8l2.3-2.3 2.3 2.3" /> : null}
+        </Icon>
+      )}
+    </button>
+  );
+}

src/components/BackupSettings.tsx

@@ -0,0 +1,146 @@
+import { useEffect, useState } from "react";
+import { Icon } from "./Icon";
+import { ConfirmDialog } from "./ConfirmDialog";
+import { useBackup } from "../store/useBackup";
+import { formatBackupTime, gdriveListBackups } from "../backup";
+
+export function BackupSettings() {
+  const open = useBackup((s) => s.settingsOpen);
+  const close = useBackup((s) => s.closeSettings);
+  const connected = useBackup((s) => s.connected);
+  const email = useBackup((s) => s.email);
+  const lastBackup = useBackup((s) => s.lastBackup);
+  const pending = useBackup((s) => s.pending);
+  const phase = useBackup((s) => s.phase);
+  const connect = useBackup((s) => s.connect);
+  const disconnect = useBackup((s) => s.disconnect);
+  const backup = useBackup((s) => s.backup);
+  const restore = useBackup((s) => s.restore);
+  const cancelConnect = useBackup((s) => s.cancelConnect);
+  const openAuthUrl = useBackup((s) => s.openAuthUrl);
+  const copyAuthUrl = useBackup((s) => s.copyAuthUrl);
+
+  const [remoteCount, setRemoteCount] = useState<number | null>(null);
+  const [confirmRestore, setConfirmRestore] = useState(false);
+  const working = phase === "working";
+
+  useEffect(() => {
+    if (!open || !connected) {
+      setRemoteCount(null);
+      return;
+    }
+    let active = true;
+    gdriveListBackups()
+      .then((list) => active && setRemoteCount(list.length))
+      .catch(() => active && setRemoteCount(null));
+    return () => {
+      active = false;
+    };
+  }, [open, connected, lastBackup]);
+
+  if (!open) return null;
+
+  return (
+    <>
+      <div className="overlay" onClick={close}>
+        <div className="panel" onClick={(e) => e.stopPropagation()}>
+          <div className="panel-head">
+            <h2>Backup &amp; Sync</h2>
+            <button className="icon-btn" onClick={close} title="Close">
+              <Icon d="M6 6l12 12M18 6L6 18" />
+            </button>
+          </div>
+          <div className="panel-body">
+            {phase === "connecting" ? (
+              <div className="backup-connecting">
+                <div className="backup-waiting">
+                  <Icon size={16}>
+                    <path d="M21 12a9 9 0 1 1-2.6-6.4M21 4v5h-5" />
+                  </Icon>
+                  <span>Waiting for Google authorization…</span>
+                </div>
+                <p>Finish in the browser tab that opened. If it didn't open, reopen the link or copy it into a browser.</p>
+                <div className="backup-link-row">
+                  <button className="backup-text-btn" onClick={cancelConnect}>
+                    Stop linking
+                  </button>
+                  <div className="backup-links">
+                    <button className="backup-text-btn" onClick={openAuthUrl}>
+                      Open link again
+                    </button>
+                    <span className="backup-link-sep">·</span>
+                    <button className="backup-text-btn" onClick={copyAuthUrl}>
+                      Copy link
+                    </button>
+                  </div>
+                </div>
+              </div>
+            ) : !connected ? (
+              <div className="backup-intro">
+                <p>
+                  Connect Google Drive to keep a private backup of every book. margin only ever sees the files it
+                  creates in a <strong>margin</strong> folder, never the rest of your Drive.
+                </p>
+                <button className="btn-primary" disabled={working} onClick={connect}>
+                  {working ? "Connecting…" : "Connect Google Drive"}
+                </button>
+              </div>
+            ) : (
+              <>
+                <div className="backup-account">
+                  <div className="backup-account-info">
+                    <span className="field-label">Connected account</span>
+                    <span className="backup-email">{email || "Google Drive"}</span>
+                  </div>
+                  <button className="btn-ghost" disabled={working} onClick={disconnect}>
+                    Disconnect
+                  </button>
+                </div>
+
+                <div className="backup-status-row">
+                  <span className={`backup-dot ${pending ? "is-pending" : "is-synced"}`} />
+                  <span>{pending ? "Changes not backed up" : formatBackupTime(lastBackup)}</span>
+                </div>
+
+                <div className="backup-actions">
+                  <button className="btn-primary" disabled={working} onClick={() => backup()}>
+                    {working ? "Working…" : "Back up now"}
+                  </button>
+                  <button
+                    className="btn-ghost"
+                    disabled={working || remoteCount === 0}
+                    onClick={() => setConfirmRestore(true)}
+                  >
+                    {remoteCount === null ? "Restore from Drive" : `Restore ${remoteCount} book${remoteCount === 1 ? "" : "s"}`}
+                  </button>
+                </div>
+
+                <p className="backup-note">
+                  Backs up automatically when you close margin, and every 15 minutes while there are changes.
+                </p>
+              </>
+            )}
+          </div>
+        </div>
+      </div>
+
+      {confirmRestore && (
+        <ConfirmDialog
+          title="Restore from Google Drive"
+          message={
+            <>
+              This replaces your local books with the copies in Google Drive. Any local changes that haven't been backed
+              up will be lost.
+            </>
+          }
+          confirmLabel="Restore"
+          onConfirm={() => {
+            setConfirmRestore(false);
+            restore();
+          }}
+          onClose={() => setConfirmRestore(false)}
+        />
+      )}
+    </>
+  );
+}

src/components/EditorView.tsx

@@ -5,6 +5,7 @@ import { Dock } from "./Dock";
 import { ResizeHandle } from "./ResizeHandle";
 import { Icon } from "./Icon";
 import { Settings } from "./Settings";
+import { BackupButton } from "./BackupButton";
 import { CoverView } from "./CoverView";
 import { FindBar } from "./FindBar";
 import { ProofPopover } from "./ProofPopover";
@@ -12,6 +13,7 @@ import { Editor } from "../editor/Editor";
 import { FloatingToolbar } from "../editor/FloatingToolbar";
 import type { ProofCoords, ProofIssue, ProofingStorage } from "../editor/proofing";
 import { COVER_ID, useBook } from "../store/useBook";
+import { useBackup } from "../store/useBackup";
 import { useProofing } from "../store/useProofing";
 import { useTheme } from "../store/useTheme";
 import { useWidth } from "../store/useWidth";
@@ -79,6 +81,7 @@ export function EditorView() {
     saveBook(current)
       .then(() => {
         if (useBook.getState().book === current) markSaved();
+        useBackup.getState().refresh();
       })
       .catch((e) => setNotice(`Save failed: ${e}`));
   }, [markSaved, setNotice]);
@@ -163,6 +166,7 @@ export function EditorView() {
           {dirty && <span className="dirty-dot" />}
         </button>
         <div className="actions">
+          {isDesktop && <BackupButton />}
           <button className="icon-btn" data-on={findOpen} onClick={() => (findOpen ? setFindOpen(false) : openFind(false))} title="Find (⌘F)">
             <Icon d="M11 4a7 7 0 1 0 0 14 7 7 0 0 0 0-14zM20 20l-4-4" />
           </button>

src/components/Library.tsx

@@ -7,13 +7,20 @@ import { isDesktop } from "../ipc";
 import { ConfirmDialog } from "./ConfirmDialog";
 import { Icon } from "./Icon";
 import { RowMenu } from "./RowMenu";
+import { BackupButton } from "./BackupButton";
+import { useBackup } from "../store/useBackup";
+import { useBook } from "../store/useBook";
 
 export function Library({ onOpen }: { onOpen: (book: Book) => void }) {
   const [books, setBooks] = useState<BookSummary[]>([]);
   const [loaded, setLoaded] = useState(false);
   const [busy, setBusy] = useState(false);
-  const [notice, setNotice] = useState<string | null>(null);
   const [pendingDelete, setPendingDelete] = useState<BookSummary | null>(null);
+  const notice = useBook((s) => s.notice);
+  const setNotice = useBook((s) => s.setNotice);
+  const restoreNonce = useBackup((s) => s.restoreNonce);
+  const restoreWorking = useBackup((s) => s.phase === "working");
+  const restoreFromDrive = useBackup((s) => s.restoreFromDrive);
 
   const refresh = () =>
     listBooks()
@@ -23,6 +30,14 @@ export function Library({ onOpen }: { onOpen: (book: Book) => void }) {
   useEffect(() => {
     refresh();
   }, []);
+  useEffect(() => {
+    if (restoreNonce) refresh();
+  }, [restoreNonce]);
+  useEffect(() => {
+    if (!notice) return;
+    const timer = setTimeout(() => setNotice(null), 4000);
+    return () => clearTimeout(timer);
+  }, [notice]);
 
   const handleExample = async () => {
     const copy = exampleBook();
@@ -64,7 +79,9 @@ export function Library({ onOpen }: { onOpen: (book: Book) => void }) {
 
   return (
     <div className="library">
-      <header className="library-head" data-tauri-drag-region />
+      <header className="library-head" data-tauri-drag-region>
+        {isDesktop && <BackupButton />}
+      </header>
       <div className="shelf">
         <button className="card card-action" onClick={() => createAndOpenBook(onOpen, setNotice)}>
           <Icon d="M12 5v14M5 12h14" size={20} />
@@ -100,6 +117,14 @@ export function Library({ onOpen }: { onOpen: (book: Book) => void }) {
           ),
         )}
       </div>
+      {loaded && books.length === 0 && isDesktop && (
+        <div className="library-restore">
+          <button className="restore-link" onClick={restoreFromDrive} disabled={restoreWorking}>
+            <Icon d="M21 12a9 9 0 1 1-2.6-6.4M21 4v5h-5" size={15} />
+            {restoreWorking ? "Restoring…" : "Restore from Google Drive"}
+          </button>
+        </div>
+      )}
       {notice && (
         <div className="toast" onClick={() => setNotice(null)}>
           {notice}

src/store/useBackup.ts

@@ -0,0 +1,156 @@
+import { create } from "zustand";
+import { openUrl } from "@tauri-apps/plugin-opener";
+import {
+  type BackupStatus,
+  gdriveBackup,
+  gdriveConnect,
+  gdriveDisconnect,
+  gdriveRestore,
+  gdriveStatus,
+} from "../backup";
+import { isDesktop } from "../ipc";
+import { useBook } from "./useBook";
+
+type Phase = "idle" | "connecting" | "working" | "error";
+
+interface BackupState {
+  connected: boolean;
+  email: string | null;
+  lastBackup: number | null;
+  pending: boolean;
+  phase: Phase;
+  error: string | null;
+  authUrl: string | null;
+  settingsOpen: boolean;
+  restoreNonce: number;
+  resolveConnect: ((ok: boolean) => void) | null;
+  apply: (status: BackupStatus) => void;
+  refresh: () => Promise<void>;
+  connect: () => Promise<boolean>;
+  cancelConnect: () => void;
+  handleAuthEvent: (ok: boolean, error: string | null) => Promise<void>;
+  openAuthUrl: () => void;
+  copyAuthUrl: () => Promise<void>;
+  disconnect: () => Promise<void>;
+  backup: (silent?: boolean) => Promise<void>;
+  restore: () => Promise<void>;
+  restoreFromDrive: () => Promise<void>;
+  openSettings: () => void;
+  closeSettings: () => void;
+}
+
+function notify(message: string) {
+  useBook.getState().setNotice(message);
+}
+
+export const useBackup = create<BackupState>((set, get) => ({
+  connected: false,
+  email: null,
+  lastBackup: null,
+  pending: false,
+  phase: "idle",
+  error: null,
+  authUrl: null,
+  settingsOpen: false,
+  restoreNonce: 0,
+  resolveConnect: null,
+  apply: (status) =>
+    set({
+      connected: status.connected,
+      email: status.email,
+      lastBackup: status.lastBackup,
+      pending: status.pending,
+    }),
+  refresh: async () => {
+    if (!isDesktop) return;
+    try {
+      get().apply(await gdriveStatus());
+    } catch {}
+  },
+  connect: () =>
+    new Promise<boolean>((resolve) => {
+      get().resolveConnect?.(false);
+      set({ phase: "connecting", error: null, authUrl: null, resolveConnect: resolve });
+      gdriveConnect()
+        .then((url) => set({ authUrl: url }))
+        .catch((e) => {
+          set({ phase: "error", error: String(e), resolveConnect: null });
+          notify(`Could not connect: ${e}`);
+          resolve(false);
+        });
+    }),
+  cancelConnect: () => {
+    const resolve = get().resolveConnect;
+    set({ phase: "idle", authUrl: null, resolveConnect: null });
+    resolve?.(false);
+  },
+  handleAuthEvent: async (ok, error) => {
+    if (get().phase !== "connecting") return;
+    const resolve = get().resolveConnect;
+    if (ok) {
+      await get().refresh();
+      set({ phase: "idle", authUrl: null, error: null, resolveConnect: null });
+      notify("Connected to Google Drive");
+    } else {
+      set({ phase: "error", authUrl: null, error: error ?? "authorization failed", resolveConnect: null });
+      notify(`Could not connect: ${error ?? "authorization failed"}`);
+    }
+    resolve?.(ok);
+  },
+  openAuthUrl: () => {
+    const url = get().authUrl;
+    if (url) openUrl(url).catch(() => {});
+  },
+  copyAuthUrl: async () => {
+    const url = get().authUrl;
+    if (!url) return;
+    try {
+      await navigator.clipboard.writeText(url);
+      notify("Authorization link copied");
+    } catch {
+      notify("Could not copy the link");
+    }
+  },
+  disconnect: async () => {
+    set({ phase: "working", error: null });
+    try {
+      get().apply(await gdriveDisconnect());
+      set({ phase: "idle" });
+      notify("Disconnected from Google Drive");
+    } catch (e) {
+      set({ phase: "error", error: String(e) });
+      notify(`Could not disconnect: ${e}`);
+    }
+  },
+  backup: async (silent = false) => {
+    if (!get().connected || get().phase === "working") return;
+    set({ phase: "working", error: null });
+    try {
+      const outcome = await gdriveBackup();
+      get().apply(outcome);
+      set({ phase: "idle" });
+      if (!silent) notify(outcome.uploaded > 0 ? "Backed up to Google Drive" : "Nothing new to back up");
+    } catch (e) {
+      set({ phase: "error", error: String(e) });
+      if (!silent) notify(`Backup failed: ${e}`);
+    }
+  },
+  restore: async () => {
+    set({ phase: "working", error: null });
+    try {
+      const result = await gdriveRestore();
+      await get().refresh();
+      set((s) => ({ phase: "idle", restoreNonce: s.restoreNonce + 1 }));
+      notify(result.restored > 0 ? `Restored ${result.restored} file${result.restored === 1 ? "" : "s"}` : "No backups found in Google Drive");
+    } catch (e) {
+      set({ phase: "error", error: String(e) });
+      notify(`Restore failed: ${e}`);
+    }
+  },
+  restoreFromDrive: async () => {
+    const ok = get().connected || (await get().connect());
+    if (ok) await get().restore();
+  },
+  openSettings: () => set({ settingsOpen: true }),
+  closeSettings: () => set({ settingsOpen: false }),
+}));

src/styles/app.css

@@ -1257,6 +1257,10 @@ body.resizing {
 
 .library-head {
   height: 44px;
+  display: flex;
+  align-items: center;
+  justify-content: flex-end;
+  padding: 0 14px;
 }
 
 .shelf {
@@ -2283,3 +2287,170 @@ body.resizing {
   border: none;
   border-radius: 0;
 }
+
+/* Google Drive backup */
+.backup-btn[data-state="pending"] {
+  color: var(--accent);
+}
+
+.backup-btn[data-state="error"] {
+  color: var(--danger);
+}
+
+.backup-btn[data-state="working"] svg {
+  animation: backup-spin 0.9s linear infinite;
+}
+
+@keyframes backup-spin {
+  to {
+    transform: rotate(360deg);
+  }
+}
+
+.library-restore {
+  display: flex;
+  justify-content: center;
+  padding: 0 44px 60px;
+}
+
+.restore-link {
+  display: inline-flex;
+  align-items: center;
+  gap: 7px;
+  color: var(--ink-faint);
+  font-family: var(--font-ui);
+  font-size: var(--t-2);
+  transition: color 120ms var(--ease);
+}
+
+.restore-link:hover:not(:disabled) {
+  color: var(--accent);
+}
+
+.restore-link:disabled {
+  opacity: 0.6;
+  cursor: default;
+}
+
+.backup-intro,
+.backup-connecting {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-start;
+  gap: 16px;
+}
+
+.backup-intro p,
+.backup-connecting p {
+  margin: 0;
+  color: var(--ink-soft);
+  font-family: var(--font-ui);
+  font-size: var(--t-3);
+  line-height: 1.55;
+}
+
+.backup-text-btn {
+  color: var(--ink-faint);
+  font-family: var(--font-ui);
+  font-size: var(--t-2);
+  transition: color 120ms var(--ease);
+}
+
+.backup-text-btn:hover {
+  color: var(--accent);
+}
+
+.backup-waiting {
+  display: flex;
+  align-items: center;
+  gap: 9px;
+  font-family: var(--font-ui);
+  font-size: var(--t-3);
+  color: var(--ink);
+}
+
+.backup-waiting svg {
+  color: var(--ink-soft);
+  animation: backup-spin 0.9s linear infinite;
+}
+
+.backup-links {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+}
+
+.backup-link-sep {
+  color: var(--line-strong);
+}
+
+.backup-link-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  width: 100%;
+  margin-top: 2px;
+}
+
+.backup-account {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 16px;
+  padding-bottom: 16px;
+  border-bottom: 1px solid var(--line);
+}
+
+.backup-account-info {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
+  min-width: 0;
+}
+
+.backup-email {
+  font-family: var(--font-ui);
+  font-size: var(--t-3);
+  color: var(--ink);
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.backup-status-row {
+  display: flex;
+  align-items: center;
+  gap: 9px;
+  margin: 18px 0;
+  font-family: var(--font-ui);
+  font-size: var(--t-3);
+  color: var(--ink-soft);
+}
+
+.backup-dot {
+  width: 8px;
+  height: 8px;
+  border-radius: 50%;
+  flex: none;
+}
+
+.backup-dot.is-synced {
+  background: var(--ink-faint);
+}
+
+.backup-dot.is-pending {
+  background: var(--accent);
+}
+
+.backup-actions {
+  display: flex;
+  gap: 10px;
+}
+
+.backup-note {
+  margin: 18px 0 0;
+  color: var(--ink-faint);
+  font-family: var(--font-ui);
+  font-size: var(--t-2);
+  line-height: 1.5;
+}