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.dart

@@ -128,6 +128,48 @@ abstract class AppLocalizations {
   /// **'Import PPT'**
   String get importPpt;
 
+  /// No description provided for @importFile.
+  ///
+  /// In en, this message translates to:
+  /// **'Import file'**
+  String get importFile;
+
+  /// No description provided for @createNotebook.
+  ///
+  /// In en, this message translates to:
+  /// **'Create notebook'**
+  String get createNotebook;
+
+  /// No description provided for @noDocumentsYet.
+  ///
+  /// In en, this message translates to:
+  /// **'No documents yet — tap Import file'**
+  String get noDocumentsYet;
+
+  /// No description provided for @processingImport.
+  ///
+  /// In en, this message translates to:
+  /// **'Importing…'**
+  String get processingImport;
+
+  /// No description provided for @importFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Couldn\'t import that file: {error}'**
+  String importFailed(String error);
+
+  /// No description provided for @convertNeedsLibreOffice.
+  ///
+  /// In en, this message translates to:
+  /// **'Importing Word documents needs LibreOffice installed. Convert to PDF first, or install LibreOffice.'**
+  String get convertNeedsLibreOffice;
+
+  /// No description provided for @unsupportedFileType.
+  ///
+  /// In en, this message translates to:
+  /// **'Unsupported file type: {ext}'**
+  String unsupportedFileType(String ext);
+
   /// No description provided for @penCanvasBeta.
   ///
   /// In en, this message translates to: