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/l10n/app_localizations.dart

@@ -140,6 +140,36 @@ abstract class AppLocalizations {
   /// **'Create notebook'**
   String get createNotebook;
 
+  /// No description provided for @newNotebookTitle.
+  ///
+  /// In en, this message translates to:
+  /// **'New notebook'**
+  String get newNotebookTitle;
+
+  /// No description provided for @notebookTitleHint.
+  ///
+  /// In en, this message translates to:
+  /// **'Notebook title'**
+  String get notebookTitleHint;
+
+  /// No description provided for @create.
+  ///
+  /// In en, this message translates to:
+  /// **'Create'**
+  String get create;
+
+  /// No description provided for @untitledNote.
+  ///
+  /// In en, this message translates to:
+  /// **'Untitled'**
+  String get untitledNote;
+
+  /// No description provided for @noNotesYetHint.
+  ///
+  /// In en, this message translates to:
+  /// **'No ink notes yet — tap + to create one'**
+  String get noNotesYetHint;
+
   /// No description provided for @noDocumentsYet.
   ///
   /// In en, this message translates to: