QR 스캔 뒤로가기 fallback 추가로 진입 복귀 보장

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

@@ -131,6 +131,15 @@ class _QRScanScreenState extends State<QRScanScreen> {
     }
   }
 
+  void _handleBack() {
+    final router = GoRouter.of(context);
+    if (router.canPop()) {
+      router.pop();
+      return;
+    }
+    router.go(buildQrBackFallbackPath());
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(
@@ -138,7 +147,7 @@ class _QRScanScreenState extends State<QRScanScreen> {
         title: Text(tr('ui.userfront.qr.title', fallback: 'Scan QR Code')),
         leading: IconButton(
           icon: const Icon(Icons.arrow_back),
-          onPressed: () => context.pop(),
+          onPressed: _handleBack,
         ),
       ),
       body: Padding(