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_zh.dart

@@ -29,6 +29,21 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get createNotebook => '新建笔记本';
 
+  @override
+  String get newNotebookTitle => '新建笔记本';
+
+  @override
+  String get notebookTitleHint => '笔记本标题';
+
+  @override
+  String get create => '创建';
+
+  @override
+  String get untitledNote => '未命名';
+
+  @override
+  String get noNotesYetHint => '还没有手写笔记——点按 + 新建';
+
   @override
   String get noDocumentsYet => '暂无文档——点按“导入文件”';