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.

lib/editor/persistence/sidecar_repository.dart

@@ -87,8 +87,18 @@ class SidecarRepository {
   /// The current in-memory sidecar (for tests / inspection).
   BadnoteSidecar get sidecar => _sidecar;
 
+  /// The standalone-notebook title loaded from the sidecar, or null.
+  String? get loadedTitle => _sidecar.title;
+
   // ── Mutations (synchronous in-memory update + debounced atomic write) ──────
 
+  /// Replace the standalone-notebook title and schedule a save. No-op if the
+  /// title is unchanged.
+  void scheduleTitleSave(String title) {
+    if (_sidecar.title == title) return;
+    _replace(title: title);
+  }
+
   /// Replace the committed strokes for [pageIndex] and schedule a save.
   void scheduleStrokeSave(int pageIndex, List<EditorStroke> strokes) {
     final next = Map<int, List<EditorStroke>>.from(_sidecar.strokes);
@@ -176,6 +186,7 @@ class SidecarRepository {
   /// given fields replaced, then arm the debounce timer. Snapshot is captured
   /// synchronously here so a later edit can't corrupt an in-flight write.
   void _replace({
+    String? title,
     Map<int, List<EditorStroke>>? strokes,
     Map<int, List<SidecarHighlight>>? highlights,
     List<SidecarScratchLink>? scratchLinks,
@@ -185,6 +196,7 @@ class SidecarRepository {
       version: _sidecar.version,
       sourceFile: _sidecar.sourceFile,
       docType: _sidecar.docType,
+      title: title ?? _sidecar.title,
       pageCount: _sidecar.pageCount,
       rotation: _sidecar.rotation,
       createdAt: _sidecar.createdAt,