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/storage/badnote_sidecar.dart

@@ -212,6 +212,7 @@ class BadnoteSidecar {
     this.version = kBadnoteSidecarVersion,
     this.sourceFile,
     this.docType,
+    this.title,
     this.pageCount,
     this.rotation = 0,
     this.createdAt,
@@ -234,6 +235,10 @@ class BadnoteSidecar {
   /// `pdf` / `pptx` / `notebook` etc.
   final String? docType;
 
+  /// Display title for a standalone (non-file-backed) notebook (`docType ==
+  /// 'notebook'`). Null for file-backed sidecars, whose title is the filename.
+  final String? title;
+
   final int? pageCount;
   final int rotation;
   final DateTime? createdAt;
@@ -252,6 +257,7 @@ class BadnoteSidecar {
         'badnoteSidecarVersion': version,
         if (sourceFile != null) 'sourceFile': sourceFile,
         if (docType != null) 'docType': docType,
+        if (title != null) 'title': title,
         if (pageCount != null) 'pageCount': pageCount,
         'rotation': rotation,
         if (createdAt != null) 'createdAt': createdAt!.toIso8601String(),
@@ -293,6 +299,7 @@ class BadnoteSidecar {
           kBadnoteSidecarVersion,
       sourceFile: json['sourceFile'] as String?,
       docType: json['docType'] as String?,
+      title: json['title'] as String?,
       pageCount: (json['pageCount'] as num?)?.toInt(),
       rotation: (json['rotation'] as num?)?.toInt() ?? 0,
       createdAt: json['createdAt'] == null