fix: 修正推荐码验证 API 路径

将 GET /user/by-referral-code/{code} 修改为正确的
GET /referrals/validate?code={code}

修复 GENESIS 推荐码验证失败 400 错误的问题。

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/mobile-app/lib/core/services/account_service.dart

@@ -1420,7 +1420,8 @@ class AccountService {
 
     try {
       final response = await _apiClient.get(
-        '/user/by-referral-code/$referralCode',
+        '/referrals/validate',
+        queryParameters: {'code': referralCode},
       );
 
       debugPrint('$_tag verifyReferralCode() - API 响应: ${response.data}');