feat(import): one Import-file entry + vault notebooks

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.

lib/l10n/app_en.arb

@@ -5,6 +5,19 @@
   "search": "Search",
   "importPdf": "Import PDF",
   "importPpt": "Import PPT",
+  "importFile": "Import file",
+  "createNotebook": "Create notebook",
+  "noDocumentsYet": "No documents yet — tap Import file",
+  "processingImport": "Importing…",
+  "importFailed": "Couldn't import that file: {error}",
+  "@importFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "convertNeedsLibreOffice": "Importing Word documents needs LibreOffice installed. Convert to PDF first, or install LibreOffice.",
+  "unsupportedFileType": "Unsupported file type: {ext}",
+  "@unsupportedFileType": {
+    "placeholders": { "ext": { "type": "String" } }
+  },
   "penCanvasBeta": "Pen Canvas (beta)",
   "newNote": "New Note",
   "open": "Open",