fix: 修复推荐码验证 API 错误处理

后端修复：
- getUserByReferralCode() 在推荐码不存在时抛出异常
- 之前返回 null，导致前端无法正确判断错误

前端修复：
- 增强 verifyReferralCode() 的响应检查
- 检查 response.data 和 data 字段是否为 null
- 添加详细的 debug 日志输出
- null 响应视为推荐码无效并抛出异常

问题原因：
- 后端返回 null 时，前端解析 response.data.data 会得到 null
- 前端没有检查 null 情况，导致显示 "status: null" 错误

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

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

backend/services/identity-service/src/application/services/user-application.service.ts

@@ -779,7 +779,9 @@ export class UserApplicationService {
     const account = await this.userRepository.findByReferralCode(
       ReferralCode.create(query.referralCode),
     );
-    if (!account) return null;
+    if (!account) {
+      throw new ApplicationError('推荐码不存在或无效');
+    }
 
     return {
       userId: account.userId.toString(),

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

@@ -1422,13 +1422,27 @@ class AccountService {
       final response = await _apiClient.get(
         '/user/by-referral-code/$referralCode',
       );
-      debugPrint('$_tag verifyReferralCode() - 推荐码有效');
 
-      final responseData = response.data as Map<String, dynamic>;
-      return responseData['data'] as Map<String, dynamic>? ?? responseData;
+      debugPrint('$_tag verifyReferralCode() - API 响应: ${response.data}');
+
+      // 检查响应数据
+      final responseData = response.data as Map<String, dynamic>?;
+      if (responseData == null) {
+        debugPrint('$_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>;
     } on ApiException catch (e) {
-      debugPrint('$_tag verifyReferralCode() - 推荐码无效: $e');
-      throw ApiException('推荐码不存在或无效');
+      debugPrint('$_tag verifyReferralCode() - API 异常: $e');
+      rethrow;
     } catch (e, stackTrace) {
       debugPrint('$_tag verifyReferralCode() - 未知异常: $e');
       debugPrint('$_tag verifyReferralCode() - 堆栈: $stackTrace');