uf 액션 토스트 공통화 및 SnackBar 제거

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

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:go_router/go_router.dart';
 import 'package:userfront/i18n.dart';
+import 'package:userfront/core/ui/toast_service.dart';
 
 import 'qr_scan_route.dart';
 
@@ -23,15 +24,8 @@ class _QRScanScreenState extends State<QRScanScreen> {
   void _submit() {
     final raw = _controller.text.trim();
     if (raw.isEmpty) {
-      ScaffoldMessenger.of(context).showSnackBar(
-        SnackBar(
-          content: Text(
-            tr(
-              'msg.userfront.qr.permission_required',
-              fallback: '카메라 권한이 필요합니다.',
-            ),
-          ),
-        ),
+      ToastService.info(
+        tr('msg.userfront.qr.permission_required', fallback: '카메라 권한이 필요합니다.'),
       );
       return;
     }