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.