fix: improve transfer recognition and local data safety

frontend/lib/shared/services/local_database.dart

@@ -1,4 +1,5 @@
 import 'dart:convert';
+import 'dart:io';
 import 'dart:math';
 
 import 'package:flutter/foundation.dart';
@@ -28,6 +29,13 @@ class LocalDatabase {
   String? _namespace;
 
   String? get namespace => _namespace;
+
+  Future<bool> namespaceExists(String namespace) async {
+    final safeNamespace = namespace.replaceAll(RegExp(r'[^a-zA-Z0-9_-]'), '_');
+    final directory = await getApplicationSupportDirectory();
+    return File(p.join(directory.path, 'jizhi_$safeNamespace.db')).exists();
+  }
+
   Database get _db {
     final database = _database;
     if (database == null) throw StateError('本地数据库尚未初始化');
@@ -766,9 +774,16 @@ class LocalDatabase {
 
   void replaceLocalTransaction(int localId, Map<String, dynamic> remote) {
     final remoteId = (remote['id'] as num).toInt();
-    _db.execute('DELETE FROM transactions WHERE id = ?', [localId]);
-    _saveIdMap('transaction', localId, remoteId);
-    cacheTransaction(remote);
+    _db.execute('BEGIN');
+    try {
+      cacheTransaction(remote);
+      _saveIdMap('transaction', localId, remoteId);
+      _db.execute('DELETE FROM transactions WHERE id = ?', [localId]);
+      _db.execute('COMMIT');
+    } catch (_) {
+      _db.execute('ROLLBACK');
+      rethrow;
+    }
   }
 
   void enqueueSync(