fix(app): 家长同意验证流程 — PIPL 第28条合规

- 新增 ParentalConsentPage: 显示隐私政策要点 + 双重确认复选框
- 角色选择流程: 学生 → 家长同意确认 → 班级码输入
- Authenticated 状态: 添加 needsParentalConsent/parentalConsentAt/selectedRole
- ParentalConsentAccepted 事件: 记录同意时间戳
- 路由守卫: 注册 /parental-consent 路径和重定向逻辑
- 非学生角色（老师/家长/独立用户）不需要经过同意流程

审计 ID: S-03

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

@@ -34,6 +34,7 @@ class AuthBloc extends Bloc<AuthEvent, AuthState> {
     on<LoginRequested>(_onLoginRequested);
     on<RegisterRequested>(_onRegisterRequested);
     on<RoleSelected>(_onRoleSelected);
+    on<ParentalConsentAccepted>(_onParentalConsentAccepted);
     on<ClassCodeSubmitted>(_onClassCodeSubmitted);
     on<LogoutRequested>(_onLogoutRequested);
     on<TokenRefreshed>(_onTokenRefreshed);
@@ -124,16 +125,38 @@ class AuthBloc extends Bloc<AuthEvent, AuthState> {
     final currentState = state;
     if (currentState is! Authenticated) return;
 
+    // 学生角色需要先经过家长同意确认（PIPL 第28条）
+    final needsParentalConsent = event.role == UserRoleType.student;
+
     // 根据角色决定下一步
     final needsClassCode =
         event.role == UserRoleType.student || event.role == UserRoleType.parent;
 
     emit(currentState.copyWith(
       needsRoleSelection: false,
-      needsClassCode: needsClassCode,
+      needsParentalConsent: needsParentalConsent,
+      needsClassCode: needsClassCode && !needsParentalConsent,
+      selectedRole: event.role,
     ));
 
-    _logger.i('角色选择: ${event.role.name}, 需要班级码: $needsClassCode');
+    _logger.i('角色选择: ${event.role.name}, 需要家长同意: $needsParentalConsent');
+  }
+
+  /// 家长/监护人同意信息收集（PIPL 合规）
+  Future<void> _onParentalConsentAccepted(
+    ParentalConsentAccepted event,
+    Emitter<AuthState> emit,
+  ) async {
+    final currentState = state;
+    if (currentState is! Authenticated) return;
+
+    _logger.i('家长同意已确认: ${event.consentAt}');
+
+    emit(currentState.copyWith(
+      needsParentalConsent: false,
+      needsClassCode: true,
+      parentalConsentAt: event.consentAt,
+    ));
   }
 
   /// 班级码加入