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.

test/vault_service_test.dart

@@ -9,6 +9,7 @@ import 'package:flutter_test/flutter_test.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
 import 'package:badnote/services/vault_service.dart';
+import 'package:badnote/storage/sidecar_store.dart';
 
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
@@ -213,4 +214,129 @@ void main() {
       }
     });
   });
+
+  group('createEmptyNotebook + scanNotes (Phase 4 standalone notebooks)', () {
+    test('writes notebook.badnote.json with the title and docType notebook',
+        () async {
+      final vault = await makeService();
+      await vault.setVaultRoot(tempDir.path);
+      final sep = Platform.pathSeparator;
+
+      final notePath = await vault.createEmptyNotebook('My Algebra Notes');
+
+      // Synthetic note path is <folder>/notebook, folder named from the title.
+      final expectedFolder = '${tempDir.path}${sep}My Algebra Notes';
+      expect(notePath, '$expectedFolder${sep}notebook');
+
+      // The sidecar exists at <folder>/notebook.badnote.json and carries the
+      // title + docType, but NO fake source file was created.
+      final sidecarFile = File('$notePath.badnote.json');
+      expect(sidecarFile.existsSync(), isTrue);
+      final loaded =
+          await SidecarStore.read(sidecarFile);
+      expect(loaded, isNotNull);
+      expect(loaded!.title, 'My Algebra Notes');
+      expect(loaded.docType, 'notebook');
+      // The folder holds only the sidecar (and possibly its .bak/.tmp), never
+      // an importable source file.
+      final files = Directory(expectedFolder)
+          .listSync()
+          .whereType<File>()
+          .map((f) => f.uri.pathSegments.last)
+          .toList();
+      expect(files, contains('notebook.badnote.json'));
+      expect(
+        files.any((n) =>
+            n.endsWith('.pdf') ||
+            n.endsWith('.pptx') ||
+            n.endsWith('.docx') ||
+            n.endsWith('.ppt')),
+        isFalse,
+      );
+    });
+
+    test('de-duplicates the notebook folder name on title collision', () async {
+      final vault = await makeService();
+      await vault.setVaultRoot(tempDir.path);
+      final sep = Platform.pathSeparator;
+
+      final a = await vault.createEmptyNotebook('Journal');
+      final b = await vault.createEmptyNotebook('Journal');
+      expect(a, '${tempDir.path}${sep}Journal${sep}notebook');
+      expect(b, '${tempDir.path}${sep}Journal 2${sep}notebook');
+    });
+
+    test('blank title falls back to an Untitled folder, null sidecar title',
+        () async {
+      final vault = await makeService();
+      await vault.setVaultRoot(tempDir.path);
+      final sep = Platform.pathSeparator;
+
+      final notePath = await vault.createEmptyNotebook('   ');
+      expect(notePath, '${tempDir.path}${sep}Untitled${sep}notebook');
+      final loaded = await SidecarStore.read(File('$notePath.badnote.json'));
+      expect(loaded!.title, isNull);
+    });
+
+    test('scanNotes lists note folders; scanNotebooks excludes them', () async {
+      final vault = await makeService();
+      await vault.setVaultRoot(tempDir.path);
+      final sep = Platform.pathSeparator;
+
+      // A standalone note.
+      final notePath = await vault.createEmptyNotebook('Ideas');
+      // A file-backed document notebook.
+      final docFolder = '${tempDir.path}${sep}Lecture';
+      final doc = File('$docFolder${sep}Lecture.pdf');
+      await doc.create(recursive: true);
+      await doc.writeAsString('pdf');
+
+      final notes = await vault.scanNotes();
+      expect(notes.length, 1, reason: 'only the standalone note is a note');
+      expect(notes.first.title, 'Ideas');
+      expect(notes.first.notePath, notePath);
+      expect(notes.first.folderPath, '${tempDir.path}${sep}Ideas');
+
+      // The document notebook is NOT a note...
+      expect(notes.any((n) => n.folderPath == docFolder), isFalse);
+      // ...and the standalone note is NOT a document.
+      final notebooks = await vault.scanNotebooks();
+      expect(notebooks.length, 1);
+      expect(notebooks.first.filename, 'Lecture.pdf');
+      expect(
+        notebooks.any((nb) => nb.folderPath == '${tempDir.path}${sep}Ideas'),
+        isFalse,
+      );
+    });
+
+    test('note title round-trips through the sidecar (title persists)',
+        () async {
+      final vault = await makeService();
+      await vault.setVaultRoot(tempDir.path);
+
+      final notePath = await vault.createEmptyNotebook('Round Trip');
+      final notes = await vault.scanNotes();
+      expect(notes.single.title, 'Round Trip');
+
+      // Re-open the sidecar directly: the title is still there.
+      final loaded = await SidecarStore.read(File('$notePath.badnote.json'));
+      expect(loaded!.title, 'Round Trip');
+    });
+
+    test('a note folder whose sidecar title is missing falls back to folder',
+        () async {
+      final vault = await makeService();
+      await vault.setVaultRoot(tempDir.path);
+      final sep = Platform.pathSeparator;
+
+      // Hand-write a note sidecar with no title field.
+      final folder = '${tempDir.path}${sep}Loose Note';
+      final sidecar = File('$folder${sep}notebook.badnote.json');
+      await sidecar.create(recursive: true);
+      await sidecar.writeAsString('{"docType":"notebook"}');
+
+      final notes = await vault.scanNotes();
+      expect(notes.single.title, 'Loose Note');
+    });
+  });
 }