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}",