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_zh.arb

@@ -7,6 +7,11 @@
   "importPpt": "导入 PPT",
   "importFile": "导入文件",
   "createNotebook": "新建笔记本",
+  "newNotebookTitle": "新建笔记本",
+  "notebookTitleHint": "笔记本标题",
+  "create": "创建",
+  "untitledNote": "未命名",
+  "noNotesYetHint": "还没有手写笔记——点按 + 新建",
   "noDocumentsYet": "暂无文档——点按“导入文件”",
   "processingImport": "正在导入…",
   "importFailed": "无法导入该文件：{error}",