feat(storage): notes are vault sidecar notebooks

Phase 4. Standalone notes move off SQLite into the vault, like the
PDF annotations.

- "Create notebook" makes a vault folder with a notebook.badnote.json
  (BadnoteSidecar docType 'notebook' + a title field), opened via
  SidecarRepository.
- PenNoteScreen loads/saves its strokes (page 0) + title to that
  sidecar instead of the SQLite Note model.
- note_provider lists notes from a vault scan (VaultService.scanNotes
  = folders with notebook.badnote.json and no source file); the doc
  scan still excludes them. Delete removes the folder.

PDF/slide editors unchanged; pre-existing SQLite notes migrate in
Phase 5. analyze clean, tests green.

test/sidecar_repository_test.dart

@@ -200,4 +200,58 @@ void main() {
     expect(sl.scratchpad.strokes.single.id, 's');
     after.dispose();
   });
+
+  group('Phase 4 standalone notebook (notebook.badnote.json)', () {
+    late String notePath;
+
+    setUp(() {
+      // The synthetic note "source" is <folder>/notebook → sidecar is
+      // <folder>/notebook.badnote.json (no real source file on disk).
+      notePath = '${tmpDir.path}/notebook';
+    });
+
+    test('a note\'s strokes (page 0) + title round-trip through the sidecar',
+        () async {
+      final repo =
+          await SidecarRepository.open(notePath, debounce: _fast, docType: 'notebook');
+      expect(repo.sidecarFile.path, '$notePath.badnote.json');
+
+      repo.scheduleTitleSave('My Note');
+      repo.scheduleStrokeSave(0, [
+        _stroke('p', tool: EditorTool.pen),
+        _stroke('h', tool: EditorTool.highlighter),
+      ]);
+      await repo.flush();
+      repo.dispose();
+
+      // Re-open the same standalone notebook sidecar.
+      final reopened =
+          await SidecarRepository.open(notePath, debounce: _fast, docType: 'notebook');
+      expect(reopened.loadedTitle, 'My Note');
+      expect(reopened.sidecar.docType, 'notebook');
+      final page0 = reopened.loadedStrokes[0]!;
+      expect(page0.map((s) => s.id), ['p', 'h']);
+      expect(page0.first.tool, EditorTool.pen);
+      expect(page0.last.tool, EditorTool.highlighter);
+      expect(page0.first.color, 0xFF112233);
+      reopened.dispose();
+    });
+
+    test('editing the title persists; unchanged title is a no-op', () async {
+      final repo =
+          await SidecarRepository.open(notePath, debounce: _fast, docType: 'notebook');
+      repo.scheduleTitleSave('First');
+      await repo.flush();
+      // Same title again: no write needed, but flush stays safe.
+      repo.scheduleTitleSave('First');
+      repo.scheduleTitleSave('Second');
+      await repo.flush();
+      repo.dispose();
+
+      final reopened =
+          await SidecarRepository.open(notePath, debounce: _fast, docType: 'notebook');
+      expect(reopened.loadedTitle, 'Second');
+      reopened.dispose();
+    });
+  });
 }