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/math-rendering.test.js

@@ -65,8 +65,7 @@ describe('KaTeX math rendering pipeline', () => {
   });
 
   test('escaped delimiters \\[...\\] and \\(...\\) both render', () => {
-    container.innerHTML =
-      '<p>Display form: \\[E = mc^2\\] and inline \\(\\pi r^2\\).</p>';
+    container.innerHTML = '<p>Display form: \\[E = mc^2\\] and inline \\(\\pi r^2\\).</p>';
 
     expect(() => renderMathInElement(container, { delimiters: DELIMITERS })).not.toThrow();
 
@@ -177,4 +176,4 @@ describe('KaTeX bundle — renderer wiring contract', () => {
     expect(src).toContain("left: '\\\\['");
     expect(src).toContain("left: '\\\\('");
   });
-});
+});