feat(client): 应用内注册新门店（两栏品牌页 + 表单）

- 新增 RegisterScreen（左品牌面板 3 步引导 + 右表单：门店名/电话/地址/
  管理员/账号/密码/确认 + 同意条款），还原原型两栏设计，全 token 化
- AuthRepository.register → POST /api/v1/register（后端已有），成功弹门店编号
  引导去登录
- 路由加 /register（公开路由）；登录页「前往官网注册」→ 应用内「立即注册」
- 整屏 golden ×三主题入回归闸
（购买流/通知中心按指示暂不做）

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TSKEiHsvauyxYUW2itzUXX

client/lib/repositories/auth_repository.dart

@@ -68,6 +68,38 @@ class AuthRepository {
     }
   }
 
+  /// POST /api/v1/register —— 自助注册新门店（公开，无需认证）。
+  /// Response: { data: { shop_code, shop_name, username } }
+  static Future<({String shopCode, String shopName, String username})>
+      register({
+    required String shopName,
+    required String phone,
+    required String address,
+    required String managerName,
+    required String username,
+    required String password,
+  }) async {
+    try {
+      final resp = await PublicApiClient.post('/register', data: {
+        'shop_name': shopName,
+        'phone': phone,
+        'address': address,
+        'manager_name': managerName,
+        'username': username,
+        'password': password,
+      });
+      final data = resp.data['data'] as Map<String, dynamic>;
+      return (
+        shopCode: data['shop_code'] as String? ?? '',
+        shopName: data['shop_name'] as String? ?? shopName,
+        username: data['username'] as String? ?? username,
+      );
+    } on DioException catch (e) {
+      final msg = e.response?.data?['error'] as String?;
+      throw AuthException(msg ?? _networkError(e));
+    }
+  }
+
   static String _networkError(DioException e) {
     switch (e.type) {
       case DioExceptionType.connectionTimeout: