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/main/PDFOperations.test.js

@@ -48,19 +48,15 @@ describe('PDFOperations - Task 15 new operations', () => {
   });
 
   describe('pdfExtractText', () => {
-    it(
-      'extracts text from all pages',
-      async () => {
-        const result = await PDFOperations.pdfExtractText({ inputPath });
+    it('extracts text from all pages', async () => {
+      const result = await PDFOperations.pdfExtractText({ inputPath });
 
-        expect(result.success).toBe(true);
-        expect(result.text).toContain('Hello Task 15 Page One');
-        expect(result.text).toContain('Second Page Content');
-      },
-      // First pdfjs-dist legacy import can exceed the 5s default on slower
-      // CI runners (observed on windows-latest)
-      30000
-    );
+      expect(result.success).toBe(true);
+      expect(result.text).toContain('Hello Task 15 Page One');
+      expect(result.text).toContain('Second Page Content');
+    }, // First pdfjs-dist legacy import can exceed the 5s default on slower
+    // CI runners (observed on windows-latest)
+    30000);
 
     it('returns failure for a nonexistent file', async () => {
       const result = await PDFOperations.pdfExtractText({
@@ -157,42 +153,38 @@ describe('PDFOperations - Task 15 new operations', () => {
   });
 
   describe('pdfExtractImages', () => {
-    it(
-      'extracts embedded raster images as PNG files',
-      async () => {
-        const imgPath = path.join(tmpDir, 'red.png');
-        await sharp({
-          create: { width: 20, height: 20, channels: 3, background: { r: 255, g: 0, b: 0 } },
-        })
-          .png()
-          .toFile(imgPath);
+    it('extracts embedded raster images as PNG files', async () => {
+      const imgPath = path.join(tmpDir, 'red.png');
+      await sharp({
+        create: { width: 20, height: 20, channels: 3, background: { r: 255, g: 0, b: 0 } },
+      })
+        .png()
+        .toFile(imgPath);
 
-        const doc = await PDFDocument.create();
-        const page = doc.addPage([300, 300]);
-        const pngImage = await doc.embedPng(fs.readFileSync(imgPath));
-        page.drawImage(pngImage, { x: 50, y: 50, width: 100, height: 100 });
+      const doc = await PDFDocument.create();
+      const page = doc.addPage([300, 300]);
+      const pngImage = await doc.embedPng(fs.readFileSync(imgPath));
+      page.drawImage(pngImage, { x: 50, y: 50, width: 100, height: 100 });
 
-        const imagePdfPath = path.join(tmpDir, 'with-image.pdf');
-        fs.writeFileSync(imagePdfPath, await doc.save());
+      const imagePdfPath = path.join(tmpDir, 'with-image.pdf');
+      fs.writeFileSync(imagePdfPath, await doc.save());
 
-        const outputDir = path.join(tmpDir, 'extracted');
-        const result = await PDFOperations.pdfExtractImages({
-          inputPath: imagePdfPath,
-          outputDir,
-        });
+      const outputDir = path.join(tmpDir, 'extracted');
+      const result = await PDFOperations.pdfExtractImages({
+        inputPath: imagePdfPath,
+        outputDir,
+      });
 
-        expect(result.success).toBe(true);
-        expect(result.count).toBeGreaterThanOrEqual(1);
-        expect(result.files.length).toBe(result.count);
+      expect(result.success).toBe(true);
+      expect(result.count).toBeGreaterThanOrEqual(1);
+      expect(result.files.length).toBe(result.count);
 
-        for (const file of result.files) {
-          expect(fs.existsSync(file)).toBe(true);
-          const meta = await sharp(file).metadata();
-          expect(meta.format).toBe('png');
-        }
-      },
-      30000 // pdfjs import + sharp decode on slower CI runners
-    );
+      for (const file of result.files) {
+        expect(fs.existsSync(file)).toBe(true);
+        const meta = await sharp(file).metadata();
+        expect(meta.format).toBe('png');
+      }
+    }, 30000); // pdfjs import + sharp decode on slower CI runners
 
     it('returns zero images for a text-only PDF', async () => {
       const outputDir = path.join(tmpDir, 'extracted-none');