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/providers/document_provider.dart

@@ -1,61 +1,68 @@
+import 'dart:io';
+
 import 'package:flutter_riverpod/flutter_riverpod.dart';
-import 'package:uuid/uuid.dart';
 
 import '../models/document.dart';
-import '../services/database_service.dart';
-import 'note_provider.dart';
+import '../services/vault_service.dart';
 
-const _uuid = Uuid();
+final vaultServiceProvider = FutureProvider<VaultService>((ref) async {
+  return VaultService.getInstance();
+});
 
 final documentListProvider =
     AsyncNotifierProvider<DocumentListNotifier, List<Document>>(
       DocumentListNotifier.new,
     );
 
+/// The home-screen document list is now sourced from a VAULT SCAN (folders
+/// under the vault root containing a source file + optional sidecar), NOT the
+/// SQLite `documents` table. The sidecar that travels with the file is the
+/// source of truth; there is no SQLite cache for this list (the scan is cheap —
+/// one directory listing — and always correct).
 class DocumentListNotifier extends AsyncNotifier<List<Document>> {
-  Future<DatabaseService> get _db => ref.read(databaseServiceProvider.future);
+  Future<VaultService> get _vault =>
+      ref.read(vaultServiceProvider.future);
 
   @override
   Future<List<Document>> build() async {
-    final db = await _db;
-    return db.getAllDocuments();
+    return _scan();
   }
 
-  /// Reloads documents from the database and publishes the result to [state]
-  /// so the UI rebuilds. Used by pull-to-refresh.
+  Future<List<Document>> _scan() async {
+    final vault = await _vault;
+    final notebooks = await vault.scanNotebooks();
+    return notebooks.map(_toDocument).toList();
+  }
+
+  /// Adapt a scanned [VaultNotebook] into the [Document] shape the home-screen
+  /// tiles already render. The notebook folder path doubles as a stable id.
+  Document _toDocument(VaultNotebook nb) {
+    return Document(
+      id: nb.folderPath,
+      filename: nb.filename,
+      docType: nb.docType,
+      filePath: nb.sourceFilePath,
+      pageCount: 0,
+      createdAt: nb.modified,
+      updatedAt: nb.modified,
+    );
+  }
+
+  /// Re-scan the vault and publish the result. Used by pull-to-refresh and
+  /// after an import.
   Future<void> loadDocuments() async {
     state = const AsyncLoading();
-    state = await AsyncValue.guard(() async {
-      final db = await _db;
-      return db.getAllDocuments();
-    });
-  }
-
-  Future<Document> addDocument({
-    required String filename,
-    required String docType,
-    required String filePath,
-    int pageCount = 0,
-  }) async {
-    final db = await _db;
-    final now = DateTime.now();
-    final document = Document(
-      id: _uuid.v4(),
-      filename: filename,
-      docType: docType,
-      filePath: filePath,
-      pageCount: pageCount,
-      createdAt: now,
-      updatedAt: now,
-    );
-    await db.insertDocument(document);
-    state = AsyncData([document, ...state.value ?? []]);
-    return document;
+    state = await AsyncValue.guard(_scan);
   }
 
+  /// Remove a notebook by deleting its folder (source file + sidecar travel
+  /// together, so removing the folder removes the whole notebook). [id] is the
+  /// notebook folder path produced by [_toDocument].
   Future<void> removeDocument(String id) async {
-    final db = await _db;
-    await db.deleteDocument(id);
+    final dir = Directory(id);
+    if (await dir.exists()) {
+      await dir.delete(recursive: true);
+    }
     final current = state.value ?? [];
     state = AsyncData(current.where((d) => d.id != id).toList());
   }