fix(payment-password): 修复 isPaymentPasswordSet 响应解析错误

API 返回 { success: true, data: { isSet: true } }，
但代码读的是 response.data['isSet']（顶层），
实际应该读 response.data['data']['isSet']（嵌套在 data 下）。
导致 isSet 永远为 null == true → false，
支付密码页面始终显示「设置」模式而非「修改」模式。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -2136,7 +2136,7 @@ class AccountService {
     debugPrint('$_tag isPaymentPasswordSet() - 查询支付密码状态');
     try {
       final response = await _apiClient.get('/user/payment-password-status');
-      final isSet = response.data['isSet'] == true;
+      final isSet = response.data['data']['isSet'] == true;
       debugPrint('$_tag isPaymentPasswordSet() - 结果: $isSet');
       return isSet;
     } catch (e) {