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_en.arb

@@ -7,6 +7,11 @@
   "importPpt": "Import PPT",
   "importFile": "Import file",
   "createNotebook": "Create notebook",
+  "newNotebookTitle": "New notebook",
+  "notebookTitleHint": "Notebook title",
+  "create": "Create",
+  "untitledNote": "Untitled",
+  "noNotesYetHint": "No ink notes yet — tap + to create one",
   "noDocumentsYet": "No documents yet — tap Import file",
   "processingImport": "Importing…",
   "importFailed": "Couldn't import that file: {error}",

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:

lib/l10n/app_localizations_en.dart

@@ -29,6 +29,21 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get createNotebook => 'Create notebook';
 
+  @override
+  String get newNotebookTitle => 'New notebook';
+
+  @override
+  String get notebookTitleHint => 'Notebook title';
+
+  @override
+  String get create => 'Create';
+
+  @override
+  String get untitledNote => 'Untitled';
+
+  @override
+  String get noNotesYetHint => 'No ink notes yet — tap + to create one';
+
   @override
   String get noDocumentsYet => 'No documents yet — tap Import file';
 

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”';
 

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}",