feat(recovery): autosave buffer + crash-recovery banner

VersionHistory snapshots the previous content on every explicit save — an
unsaved buffer is still lost on crash. AutosaveBuffer fills that gap.

- src/main/AutosaveBuffer.js — pure module mirroring VersionHistory's
  injectable-IO pattern; one blob per doc path under
  <userData>/autosave/by-path/<sha1>/recovery.md + meta.json. No history
  (VersionHistory owns that) — just the latest dirty buffer.
- src/main.js — IPC channels autosave:write/read/clear/list; real paths
  go through validatePath, synthetic 'untitled-tab-<id>' keys skip it.
- src/preload.js — added the four channels to ALLOWED_SEND_CHANNELS.
- src/renderer/autosave-client.js — debounced (2s) flush per tab +
  periodic safety net (10s max age) + dirty-write retry on failure.
- src/renderer.js — register on tab create, unregister on close,
  notifyChange piggybacks on performAutoSave's existing dirty-check,
  clearForDocPath after a successful save, showAutosaveRecoveryBanner
  on startup listing pending recoveries with Restore/Dismiss.

Tests (39 new):
- tests/main/AutosaveBuffer.test.js (19): round-trip, overwrite, isolation,
  unicode/emoji, empty content, null coercion, ENOENT vs corrupt meta,
  list ordering, corrupt-sibling skip, input validation, sha1 storage.
- tests/autosave-client.test.js (11): debounce, flushNow bypass,
  no-path skip, clearForDocPath, list proxy, IPC error fallback,
  unregister tear-down, failure-retry, periodic flush, idempotency.

Full suite: 692 tests pass, 63 suites, lint+format clean.

Amit Haridas

tests/bundled-tools.test.js

@@ -64,14 +64,27 @@ describe('Legal compliance artifacts', () => {
   });
 
   test('canonical copyleft license texts are present', () => {
-    for (const file of ['GPL-2.0.txt', 'LGPL-2.1.txt', 'MPL-2.0.txt', 'Apache-2.0.txt', 'OFL-1.1.txt']) {
+    for (const file of [
+      'GPL-2.0.txt',
+      'LGPL-2.1.txt',
+      'MPL-2.0.txt',
+      'Apache-2.0.txt',
+      'OFL-1.1.txt',
+    ]) {
       expect(fs.existsSync(path.join(rootDir, 'third-party-licenses', file))).toBe(true);
     }
   });
 
   test('notices mention every bundled external binary', () => {
     const notices = fs.readFileSync(path.join(rootDir, 'THIRD-PARTY-NOTICES.md'), 'utf-8');
-    for (const component of ['Pandoc', 'FFmpeg', 'MarkItDown', 'libvips', 'KaTeX', 'JetBrains Mono']) {
+    for (const component of [
+      'Pandoc',
+      'FFmpeg',
+      'MarkItDown',
+      'libvips',
+      'KaTeX',
+      'JetBrains Mono',
+    ]) {
       expect(notices).toContain(component);
     }
     // GPL source offer must reference the sources document
@@ -90,10 +103,7 @@ describe('Legal compliance artifacts', () => {
   test('packaging includes the legal documents in build files', () => {
     // The build config lives in electron-builder.config.js (dynamic — it
     // conditionally bundles the markitdown binary per platform)
-    const configSrc = fs.readFileSync(
-      path.join(rootDir, 'electron-builder.config.js'),
-      'utf-8'
-    );
+    const configSrc = fs.readFileSync(path.join(rootDir, 'electron-builder.config.js'), 'utf-8');
     expect(configSrc).toContain("'THIRD-PARTY-NOTICES.md'");
     expect(configSrc).toContain("'SOURCES.md'");
     expect(configSrc).toContain('third-party-licenses');