fix: harden iOS authentication recovery

mobile/lib/core/auth/auth_session_controller.dart

@@ -19,11 +19,19 @@ class AuthSessionController extends AsyncNotifier<AuthSession?> {
   static const _appAuth = FlutterAppAuth();
 
   @override
-  Future<AuthSession?> build() => _restore();
+  Future<AuthSession?> build() async {
+    try {
+      return await _restore();
+    } catch (_) {
+      // 安全存储在模拟器重装、系统升级或钥匙串状态变化后可能暂时不可读。
+      // 将其视为无本地会话，避免阻断用户重新登录。
+      return null;
+    }
+  }
 
   Future<void> login() async {
     state = const AsyncLoading();
-    state = await AsyncValue.guard(() async {
+    try {
       final result = await _appAuth.authorizeAndExchangeCode(
         AuthorizationTokenRequest(
           oidcClientId,
@@ -34,8 +42,12 @@ class AuthSessionController extends AsyncNotifier<AuthSession?> {
           allowInsecureConnections: oidcIssuer.startsWith('http://'),
         ),
       );
-      return _storeResponse(result);
-    });
+      state = AsyncData(await _storeResponse(result));
+    } on FlutterAppAuthUserCancelledException {
+      state = const AsyncData(null);
+    } catch (error, stackTrace) {
+      state = AsyncError(error, stackTrace);
+    }
   }
 
   Future<void> logout() async {

mobile/lib/core/auth/login_page.dart

@@ -39,7 +39,7 @@ class LoginPage extends ConsumerWidget {
                     if (auth.hasError) ...[
                       const SizedBox(height: 16),
                       Text(
-                        '登录失败：${auth.error}',
+                        '登录失败，请检查网络后重试',
                         textAlign: TextAlign.center,
                         style: TextStyle(
                           color: Theme.of(context).colorScheme.error,