Margin Calendar: a Google Calendar client for desktop and phone

Tauri 2, React 19 and zustand on the front, Rust behind. Rust owns auth,
all HTTP to Google, the SQLite store, the sync loop, recurrence expansion
and timezone maths. TypeScript owns rendering and never talks to Google,
which keeps the content security policy locked to ipc:.

Week, day and agenda views, and no month view: it would be a second layout
engine, and the fit and fold logic that makes a day fit the window without
scrolling is the whole point of the app.

Runs on macOS, Linux, Android and iOS. Desktop catches Google's OAuth
redirect on a loopback port. A phone cannot, and Google rejects loopback
for mobile client types anyway, so it redirects to a custom URI scheme and
needs its own public OAuth clients, which docs/mobile.md covers. Refresh
tokens are sealed with XChaCha20-Poly1305 in the app data directory on
every platform, with no OS credential store in the picture.

On a phone the chrome becomes a top bar and a bottom tab bar, overlays
become sheets, hover affordances become taps, and dragging out an event
waits for a long press. Navigation moves one day at a time everywhere,
a swipe included.

docs/architecture.md

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+# Architecture
+
+Tauri 2, React 19, Vite, TypeScript and zustand on the front, Rust behind. Same stack as
+margin, which means the OAuth flow, the token handling and the build and bundle setup carry
+over rather than being invented again.
+
+The split is strict. Rust owns authentication, all HTTP to Google, the local store, the sync
+loop, recurrence expansion and timezone arithmetic. TypeScript owns rendering and interaction
+and nothing else. The frontend never talks to Google, which keeps the content security policy
+locked to `ipc:` exactly as margin has it.
+
+## Authentication
+
+Lifted from `margin/src-tauri/src/gdrive.rs`, then split in two when mobile arrived. Both halves
+build the same consent URL with PKCE S256 and a CSRF state parameter and open it in the system
+browser through the opener plugin, never an in-app webview: Google blocks the embedded-webview
+flow, and it deserves to be blocked, because a webview the app controls can read the password
+typed into it. They differ only in how the answer comes back.
+
+Desktop binds a listener on `127.0.0.1:0` and catches the redirect on the loopback socket. The
+verifier lives on that listener's stack for the two minutes it is alive.
+
+Mobile cannot do that. iOS will not keep a background listener alive dependably, and Google
+rejects loopback redirects for Android and iOS client types outright, so the redirect is a custom
+URI scheme the OS routes back to the app through `tauri-plugin-deep-link`. There is no listener to
+hold the verifier, the browser is a separate app and this process may be backgrounded while the
+user consents, so the verifier waits in `AuthState.pending` and `handle_redirect` picks it up
+whenever the link lands. It is taken rather than read, so a replayed link cannot start a second
+exchange.
+
+That split forces one more difference. A desktop client is confidential and has a secret; an
+Android or iOS client is public and has none, so `google-credentials.json` carries up to three
+clients and the build embeds the one for the platform it is compiling for. Details and the
+console steps are in [mobile.md](mobile.md).
+
+The scope is `https://www.googleapis.com/auth/calendar` plus `openid email`. Calendar is a
+sensitive scope, so an unverified client shows the unverified-app interstitial and caps at 100
+users. Irrelevant for personal use, relevant the moment this ships the way margin ships.
+
+Refresh tokens never go to disk in plaintext and never into SQLite. They are sealed with
+XChaCha20-Poly1305 in the app data directory, the same way on all five platforms.
+
+There is no `keyring` here, and its absence is deliberate rather than an omission. It has no
+Android backend at all; on macOS it ties an item to the code signature, so an ad-hoc signed build
+gets a new identity on every compile and macOS re-asks for authorization every time; and on Linux
+it is missing on exactly the minimal window managers that run no Secret Service daemon. That left
+one real implementation behind four ways of reaching it. `src-tauri/src/google/secrets.rs` states
+plainly what the file is worth on each platform, which is more on the mobile sandboxes than on a
+desktop home directory.
+
+## Store
+
+SQLite through `rusqlite` with the bundled feature, so there is no system SQLite dependency on
+either platform.
+
+Accounts hold an email and a reference naming the sealed token, never the token itself. The
+column is still called `keychain_ref` from when there was a keychain; renaming it would cost a
+migration and buy nothing. Calendars hold their account, summary,
+colour, selection state, access role, default timezone and their own sync token. Events hold
+the raw Google shape flattened: identity and etag, summary, description, location, start and
+end with their separate timezones, an all-day flag, the recurrence rule set, the pointer back
+to a master plus original start for exception instances, status, and attendees and conference
+data as JSON blobs. An outbox table holds pending writes.
+
+Events are stored as Google returns them with `singleEvents=false`, meaning recurring series
+are one row carrying an RRULE rather than thousands of rows. Expansion happens on read.
+
+## Sync
+
+Incremental sync through `events.list` with a stored `syncToken`, one cursor per calendar,
+`singleEvents=false`, `showDeleted=true`, `maxResults=2500`. The parameter set has to be
+byte-identical across every call in a chain, including the initial one.
+
+The constraint that shapes everything: `timeMin` and `timeMax` cannot be used with
+`syncToken`, along with `q`, `orderBy`, `updatedMin`, `iCalUID` and the extended property
+filters. There is no way to sync a window. The initial full sync pulls the entire calendar
+history, which for an old personal calendar is a few thousand rows and a one-time cost of
+seconds, and is only tolerable because recurring series collapse to single rows.
+
+`nextSyncToken` appears only on the final page, so pages must be walked to exhaustion with
+`pageToken` before anything is persisted. A 410 means the token is dead: drop that calendar's
+rows and its cursor and full-resync it alone, without touching the others. `calendarList.list`
+carries its own sync token for the set of calendars itself.
+
+Polling, not webhooks. Push notifications need a publicly verified HTTPS callback and channels
+that expire every few days, neither of which a desktop app has. Sixty seconds while the window
+is focused, five minutes when it is not. An incremental poll with no changes is a single small
+request against a million-a-day quota.
+
+## Writes
+
+Optimistic. A write lands in SQLite, is marked dirty, is enqueued in the outbox and renders
+immediately; the push to Google happens behind it. Requests carry `If-Match` with the stored
+etag, and a 412 means someone else changed it first, so refetch and surface the conflict
+rather than clobbering. Offline writes queue and drain on reconnect. The outbox is the reason
+the app works on a plane and the reason it never spins.
+
+## Recurrence
+
+The `rrule` crate, which is built on `chrono-tz`. Expansion takes a window and a set of
+masters, generates occurrences, drops any occurrence whose original start matches a cancelled
+or overridden instance, then merges the override events back in at their moved times. The
+frontend asks for a range and receives a flat array of instances; it has no concept of a
+recurrence rule.
+
+Editing a series is three distinct operations. Editing one instance resolves the real instance
+through `events.instances` and patches that, rather than constructing the instance id by hand.
+Editing this and all following truncates the master's rule with an UNTIL and creates a new
+master from the split point. Editing the whole series patches the master. All three are worth
+tests before they are worth UI.
+
+## Time
+
+Every event keeps its own timezone; the grid renders in the local zone. All-day events are
+date-only and must never be shifted into a zone, which is the single most common bug in this
+category of app and deserves a test that runs in a non-UTC zone. DST transitions mean a day is
+sometimes 23 or 25 hours long, which the vertical fit calculation has to accept rather than
+assume away.
+
+## Platforms
+
+macOS uses the overlay title bar with the header row padded for the traffic lights, as margin
+does. Linux has no traffic lights, so that padding is conditional. Linux builds need
+`libwebkit2gtk-4.1-dev` and ship as AppImage and deb.
+
+## Order of work
+
+The sync engine and the grid are the two hard things and neither is proven by the other, so
+the first milestone is authentication, sync and a read-only grid with correct vertical fit.
+Recurrence and timezone correctness come second, with tests, because everything after depends
+on the instance stream being right. Create, edit and delete with the outbox come third.
+Multiple accounts, the command palette and natural language parsing come last, since they are
+the parts that are pleasant rather than load-bearing.