fix: 修正推荐码验证API响应解析

- referral-service返回 {valid:bool} 格式
- 移除对 data 字段的依赖

🤖 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

@@ -1426,20 +1426,15 @@ class AccountService {
       debugPrint('$_tag verifyReferralCode() - API 响应: ${response.data}');
 
       // 检查响应数据
+      // referral-service 直接返回 {"valid": true/false} 格式
       final responseData = response.data as Map<String, dynamic>?;
       if (responseData == null) {
         debugPrint('$_tag verifyReferralCode() - 响应数据为 null');
         throw ApiException('推荐码不存在或无效');
       }
 
-      final data = responseData['data'];
-      if (data == null) {
-        debugPrint('$_tag verifyReferralCode() - data 字段为 null');
-        throw ApiException('推荐码不存在或无效');
-      }
-
-      debugPrint('$_tag verifyReferralCode() - 推荐码有效');
-      return data as Map<String, dynamic>;
+      debugPrint('$_tag verifyReferralCode() - 推荐码验证完成');
+      return responseData;
     } on ApiException catch (e) {
       debugPrint('$_tag verifyReferralCode() - API 异常: $e');
       rethrow;