feat(auth): 实现注册模块与登录页优化

1. 新增用户注册接口与页面，对齐Vue端业务逻辑
2. 新增居中Toast组件，替换原有SnackBar轻提示
3. 完善登录历史本地存储与下拉选择交互
4. 新增审核模式接口适配，动态控制注册入口显示
5. 优化登录页UI布局与交互细节

lib/core/router/app_router.dart

@@ -6,6 +6,7 @@ import 'package:new_learn/features/auth/legal_document_page.dart';
 import 'package:new_learn/features/auth/legal_documents.dart';
 import 'package:new_learn/features/auth/login_page.dart';
 import 'package:new_learn/features/auth/privacy_consent_page.dart';
+import 'package:new_learn/features/auth/register_page.dart';
 import 'package:new_learn/features/class/all_student_view.dart';
 import 'package:new_learn/features/class/class_page.dart';
 import 'package:new_learn/features/class/my_interact_page.dart';
@@ -145,8 +146,9 @@ final GoRouter appRouter = GoRouter(
         content: kPrivacyPolicyContent,
       ),
     ),
-    // ---------- 登录页 ----------
+    // ---------- 登录/注册页（未登录可访问） ----------
     GoRoute(path: '/login', builder: (_, _) => const LoginPage()),
+    GoRoute(path: '/register', builder: (_, _) => const RegisterPage()),
     // app_router.dart 的 routes 里加，不用管跳转逻辑
     // GoRoute(path: '/task/:id', builder: (_, _) => const TaskDetailPage()),
     // ---------- 以后新业务页在这里加，一行一个 ----------
@@ -154,7 +156,7 @@ final GoRouter appRouter = GoRouter(
 );
 
 /// 登录守卫（对标 vue-router beforeEach）：
-///   没登录 → 强制去 /login；已登录还访问 /login → 踢回首页
+///   没登录 → 强制去 /login；已登录还访问 /login 或 /register → 踢回首页
 String? _guardLogin(BuildContext context, GoRouterState state) {
   final acceptedPrivacy = privacyAcceptedNotifier.value;
   final loggedIn = DioClient.instance.accessToken != null;
@@ -163,11 +165,14 @@ String? _guardLogin(BuildContext context, GoRouterState state) {
       state.matchedLocation == '/user-agreement' ||
       state.matchedLocation == '/privacy-policy';
   final atLogin = state.matchedLocation == '/login';
+  final atRegister = state.matchedLocation == '/register';
+  // 未登录可白名单：隐私协议页 / 协议文档 / 登录页 / 注册页
+  final atAuthFree = atLogin || atRegister || atLegalDocument;
   if (!acceptedPrivacy && !atPrivacyConsent && !atLegalDocument) {
     return '/privacy-consent';
   }
   if (acceptedPrivacy && atPrivacyConsent) return loggedIn ? '/' : '/login';
-  if (!loggedIn && !atLogin && !atLegalDocument) return '/login';
-  if (loggedIn && atLogin) return '/';
+  if (!loggedIn && !atAuthFree && !atPrivacyConsent) return '/login';
+  if (loggedIn && (atLogin || atRegister)) return '/';
   return null;
 }