refactor: share one BeeCount Cloud provider instance

lib/providers/sync_providers.dart

@@ -141,17 +141,20 @@ final s3ConfigProvider = FutureProvider<CloudServiceConfig?>((ref) async {
 });
 
 final authServiceProvider = FutureProvider<CloudAuthService>((ref) async {
-  final activeAsync = ref.watch(activeCloudConfigProvider);
-  if (!activeAsync.hasValue) {
-    return NoopAuthService();
-  }
-
-  final config = activeAsync.value!;
+  final config = await ref.watch(activeCloudConfigProvider.future);
   if (!config.valid || config.type == CloudBackendType.local) {
     return NoopAuthService();
   }
 
   try {
+    // BeeCount Cloud 必须复用同步引擎持有的唯一 provider/auth 实例。
+    // 多个实例虽然共用 SharedPreferences，却各自缓存 session；这会让 2FA
+    // 登录成功后同步实例仍停留在未登录状态。
+    if (config.type == CloudBackendType.beecountCloud) {
+      final provider = await ref.watch(beecountCloudProviderInstance.future);
+      return provider?.auth ?? NoopAuthService();
+    }
+
     final services = await createCloudServices(config);
     if (services.auth != null) {
       return services.auth!;
@@ -488,10 +491,7 @@ final syncServiceProvider = Provider<SyncService>((ref) {
 /// 用于 SyncEngine 和其他需要直接访问 BeeCount Cloud API 的场景
 final beecountCloudProviderInstance =
     FutureProvider<BeeCountCloudProvider?>((ref) async {
-  final configAsync = ref.watch(activeCloudConfigProvider);
-  if (!configAsync.hasValue) return null;
-
-  final config = configAsync.value!;
+  final config = await ref.watch(activeCloudConfigProvider.future);
   if (!config.valid || config.type != CloudBackendType.beecountCloud) {
     return null;
   }

test/providers/beecount_cloud_auth_provider_test.dart

@@ -0,0 +1,36 @@
+import 'package:flutter_cloud_sync/flutter_cloud_sync.dart';
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:beecount/providers/sync_providers.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+  });
+
+  test('BeeCount Cloud UI auth and SyncEngine share one auth instance',
+      () async {
+    const config = CloudServiceConfig(
+      type: CloudBackendType.beecountCloud,
+      name: 'BeeCount Cloud',
+      beecountCloudBaseUrl: 'https://cloud.example.com',
+      beecountCloudApiPrefix: '/api/v1',
+    );
+    final container = ProviderContainer(
+      overrides: [
+        activeCloudConfigProvider.overrideWith((ref) async => config),
+      ],
+    );
+    addTearDown(container.dispose);
+
+    final provider = await container.read(beecountCloudProviderInstance.future);
+    final auth = await container.read(authServiceProvider.future);
+
+    expect(provider, isNotNull);
+    expect(identical(auth, provider!.auth), isTrue);
+  });
+}