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_localizations_en.dart

@@ -23,6 +23,32 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get importPpt => 'Import PPT';
 
+  @override
+  String get importFile => 'Import file';
+
+  @override
+  String get createNotebook => 'Create notebook';
+
+  @override
+  String get noDocumentsYet => 'No documents yet — tap Import file';
+
+  @override
+  String get processingImport => 'Importing…';
+
+  @override
+  String importFailed(String error) {
+    return 'Couldn\'t import that file: $error';
+  }
+
+  @override
+  String get convertNeedsLibreOffice =>
+      'Importing Word documents needs LibreOffice installed. Convert to PDF first, or install LibreOffice.';
+
+  @override
+  String unsupportedFileType(String ext) {
+    return 'Unsupported file type: $ext';
+  }
+
   @override
   String get penCanvasBeta => 'Pen Canvas (beta)';