fix: 修改推荐码验证API路径为referral-service

- 从 /referrals/validate?code=X 改为 /referral/validate/X
- 使用 referral-service 的 API (已在 Kong 配置)

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

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

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

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