Phase 3. Import becomes a single top-level action beside "Create
notebook" and the library is vault-backed.
- VaultService.createNotebook copies a picked file into a fresh
(de-duplicated) notebook folder under the vault; its sidecar lives
beside it, so annotations travel with the file.
- Home screen: one "Import file" action with a multi-extension picker
(pdf / docx / pptx); routes to the editor by extension.
- The document list is now a vault scan (folders with a source file),
not the SQLite documents table — no cache, always correct.
- PPTX soffice detection fix; DOCX convert-on-import is best-effort
and fails gracefully when LibreOffice is unavailable.
analyze clean, tests green.