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/screens/home_screen.dart

@@ -119,7 +119,7 @@ class HomeScreen extends ConsumerWidget {
                       ),
                       child: Center(
                         child: Text(
-                          'No ink notes yet — tap + to create one',
+                          l.noNotesYetHint,
                           style: Theme.of(context).textTheme.bodyMedium
                               ?.copyWith(
                                 color: Theme.of(
@@ -180,8 +180,18 @@ class HomeScreen extends ConsumerWidget {
     );
   }
 
+  /// "Create notebook": prompt a title (defaulting to Untitled), create the
+  /// standalone notebook FOLDER + `notebook.badnote.json` via
+  /// `VaultService.createEmptyNotebook`, then open the editor on the new note.
   Future<void> _createAndOpenNote(BuildContext context, WidgetRef ref) async {
-    final note = await ref.read(noteListProvider.notifier).createNote();
+    final title = await _promptNotebookTitle(context);
+    if (title == null) return; // cancelled
+    final l = context.mounted ? AppLocalizations.of(context) : null;
+    final resolved = title.trim().isEmpty
+        ? (l?.untitledNote ?? 'Untitled')
+        : title.trim();
+    final note =
+        await ref.read(noteListProvider.notifier).createNote(title: resolved);
     if (context.mounted) {
       Navigator.of(
         context,
@@ -189,6 +199,35 @@ class HomeScreen extends ConsumerWidget {
     }
   }
 
+  /// Ask for a notebook title. Returns the entered string (possibly empty →
+  /// caller defaults it), or null if the user cancelled.
+  Future<String?> _promptNotebookTitle(BuildContext context) {
+    final l = AppLocalizations.of(context);
+    final controller = TextEditingController(text: l.untitledNote);
+    return showDialog<String>(
+      context: context,
+      builder: (ctx) => AlertDialog(
+        title: Text(l.newNotebookTitle),
+        content: TextField(
+          controller: controller,
+          autofocus: true,
+          decoration: InputDecoration(hintText: l.notebookTitleHint),
+          onSubmitted: (v) => Navigator.of(ctx).pop(v),
+        ),
+        actions: [
+          TextButton(
+            onPressed: () => Navigator.of(ctx).pop(),
+            child: Text(l.cancel),
+          ),
+          TextButton(
+            onPressed: () => Navigator.of(ctx).pop(controller.text),
+            child: Text(l.create),
+          ),
+        ],
+      ),
+    );
+  }
+
   /// Single top-level "Import file" action (sibling of "Create notebook"):
   /// pick a pdf/docx/pptx/ppt, copy it into a new vault notebook folder, then
   /// open the IN-VAULT copy in the right editor (routed by extension).
@@ -386,8 +425,10 @@ class _NoteTileState extends ConsumerState<_NoteTile> {
             child: Column(
               crossAxisAlignment: CrossAxisAlignment.start,
               children: [
+                // Stroke count is no longer cached in the vault scan (strokes
+                // load lazily in the editor), so the tile shows only the date.
                 Text(
-                  '${note.strokes.length} stroke${note.strokes.length == 1 ? '' : 's'} · $dateStr',
+                  dateStr,
                   style: Theme.of(context).textTheme.bodySmall,
                 ),
                 if (note.tags.isNotEmpty)