uf 액션 토스트 공통화 및 SnackBar 제거

userfront/lib/features/auth/presentation/login_screen.dart

@@ -18,6 +18,7 @@ import '../domain/cookie_session_policy.dart';
 import '../domain/login_link_route_policy.dart';
 import '../../profile/domain/notifiers/profile_notifier.dart';
 import '../../../core/services/web_window.dart';
+import '../../../core/ui/toast_service.dart';
 
 class LoginScreen extends ConsumerStatefulWidget {
   final String? verificationToken;
@@ -1153,9 +1154,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen>
 
   void _showError(String message) {
     if (!mounted) return;
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text(message), backgroundColor: Colors.red),
-    );
+    ToastService.error(message);
     try {
       AuthProxyService.logError(message);
     } catch (e) {
@@ -1165,9 +1164,7 @@ class _LoginScreenState extends ConsumerState<LoginScreen>
 
   void _showInfo(String message) {
     if (!mounted) return;
-    ScaffoldMessenger.of(context).showSnackBar(
-      SnackBar(content: Text(message), backgroundColor: Colors.green),
-    );
+    ToastService.success(message);
   }
 
   void _logTokenDetails(String jwt) {