feat(fonts): add font picker to book setup

Add a Typography section to book setup with one-click preset pairings,
an advanced body/heading picker, and a live sample. Enumerate installed
fonts via a new list_system_fonts Tauri command (fontdb) so they appear
alongside the bundled set, with a note that system fonts may not embed
in exports.

src-tauri/src/fonts.rs

@@ -0,0 +1,47 @@
+use fontdb::{Database, Family, Query};
+
+fn system_db() -> Database {
+    let mut db = Database::new();
+    db.load_system_fonts();
+    db
+}
+
+#[tauri::command(async)]
+pub fn list_system_fonts() -> Vec<String> {
+    let db = system_db();
+    let mut names: Vec<String> = db
+        .faces()
+        .filter_map(|face| face.families.first().map(|(name, _)| name.clone()))
+        .collect();
+    names.sort();
+    names.dedup();
+    names
+}
+
+pub fn system_font_bytes(family: &str) -> Vec<Vec<u8>> {
+    let db = system_db();
+    let mut out = Vec::new();
+    let styles = [
+        (fontdb::Weight::NORMAL, fontdb::Style::Normal),
+        (fontdb::Weight::NORMAL, fontdb::Style::Italic),
+        (fontdb::Weight::BOLD, fontdb::Style::Normal),
+        (fontdb::Weight::BOLD, fontdb::Style::Italic),
+    ];
+    let mut seen = std::collections::HashSet::new();
+    for (weight, style) in styles {
+        let query = Query {
+            families: &[Family::Name(family)],
+            weight,
+            style,
+            ..Query::default()
+        };
+        if let Some(id) = db.query(&query) {
+            if seen.insert(id) {
+                if let Some(bytes) = db.with_face_data(id, |data, _index| data.to_vec()) {
+                    out.push(bytes);
+                }
+            }
+        }
+    }
+    out
+}

src-tauri/src/lib.rs

@@ -1,4 +1,5 @@
 mod epub;
+mod fonts;
 mod library;
 mod pdf;
 mod project;
@@ -83,6 +84,7 @@ pub fn run() {
         .invoke_handler(tauri::generate_handler![
             epub::package_epub,
             epub::unzip_epub,
+            fonts::list_system_fonts,
             pdf::compile_pdf,
             project::read_file,
             project::write_file,