style(userfront): add missing curly braces for flow control structures

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

@@ -1488,8 +1488,9 @@ class _SignupScreenState extends State<SignupScreen> {
                                                 .replaceAll('Exception: ', ''),
                                           );
                                         } finally {
-                                          if (mounted)
+                                          if (mounted) {
                                             setState(() => _isLoading = false);
+                                          }
                                         }
                                       },
                                 child: const Text('중복 확인'),