uf 액션 토스트 공통화 및 SnackBar 제거

userfront/lib/main.dart

@@ -28,6 +28,7 @@ import 'core/i18n/locale_gate.dart';
 import 'core/i18n/locale_registry.dart';
 import 'core/i18n/locale_utils.dart';
 import 'core/i18n/toml_asset_loader.dart';
+import 'core/ui/toast_service.dart';
 import 'package:logging/logging.dart';
 import 'features/auth/presentation/consent_screen.dart';
 import 'i18n.dart';
@@ -370,6 +371,11 @@ class BaronSSOApp extends StatelessWidget {
       localizationsDelegates: delegates,
       supportedLocales: supportedLocales,
       locale: locale,
+      builder: (context, child) {
+        return Stack(
+          children: [if (child != null) child, const ToastViewport()],
+        );
+      },
       theme: ThemeData(
         colorScheme: ColorScheme.fromSeed(
           seedColor: const Color(0xFF1A1F2C), // Dark Navy/Black base