feat(app): 班级码验证前后端联调 — AuthBloc接入API + 错误计数锁定UI

app/lib/features/auth/bloc/auth_state.dart

@@ -40,21 +40,33 @@ final class Authenticated extends AuthState {
   /// 是否需要班级码加入（学生/家长角色）
   final bool needsClassCode;
 
+  /// 是否正在加载（班级码验证中）
+  final bool isLoading;
+
+  /// 班级码验证错误信息
+  final String? classCodeError;
+
   const Authenticated({
     required this.user,
     this.needsRoleSelection = false,
     this.needsClassCode = false,
+    this.isLoading = false,
+    this.classCodeError,
   });
 
   Authenticated copyWith({
     User? user,
     bool? needsRoleSelection,
     bool? needsClassCode,
+    bool? isLoading,
+    String? classCodeError,
   }) =>
       Authenticated(
         user: user ?? this.user,
         needsRoleSelection: needsRoleSelection ?? this.needsRoleSelection,
         needsClassCode: needsClassCode ?? this.needsClassCode,
+        isLoading: isLoading ?? this.isLoading,
+        classCodeError: classCodeError,
       );
 }