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_state.dart

@@ -37,6 +37,9 @@ final class Authenticated extends AuthState {
   /// 是否需要角色选择（新注册用户还没有角色）
   final bool needsRoleSelection;
 
+  /// 是否需要家长/监护人同意（PIPL 第28条 — 学生角色）
+  final bool needsParentalConsent;
+
   /// 是否需要班级码加入（学生/家长角色）
   final bool needsClassCode;
 
@@ -46,27 +49,43 @@ final class Authenticated extends AuthState {
   /// 班级码验证错误信息
   final String? classCodeError;
 
+  /// 已选择的角色（角色选择后暂存）
+  final UserRoleType? selectedRole;
+
+  /// 家长同意时间戳
+  final DateTime? parentalConsentAt;
+
   const Authenticated({
     required this.user,
     this.needsRoleSelection = false,
+    this.needsParentalConsent = false,
     this.needsClassCode = false,
     this.isLoading = false,
     this.classCodeError,
+    this.selectedRole,
+    this.parentalConsentAt,
   });
 
   Authenticated copyWith({
     User? user,
     bool? needsRoleSelection,
+    bool? needsParentalConsent,
     bool? needsClassCode,
     bool? isLoading,
     String? classCodeError,
+    UserRoleType? selectedRole,
+    DateTime? parentalConsentAt,
   }) =>
       Authenticated(
         user: user ?? this.user,
         needsRoleSelection: needsRoleSelection ?? this.needsRoleSelection,
+        needsParentalConsent:
+            needsParentalConsent ?? this.needsParentalConsent,
         needsClassCode: needsClassCode ?? this.needsClassCode,
         isLoading: isLoading ?? this.isLoading,
         classCodeError: classCodeError,
+        selectedRole: selectedRole ?? this.selectedRole,
+        parentalConsentAt: parentalConsentAt ?? this.parentalConsentAt,
       );
 }