fix: admin page reactive import + member payment toast show real error

1. admin.vue: 添加缺失的reactive导入，修复管理后台只显示标题不显示功能的问题

2. member.vue: 支付失败toast改为显示后端真实错误信息，而非硬编码'创建订单失败'

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

zhiyin-app/src/pages/admin/admin.vue

@@ -393,7 +393,7 @@
 </template>
 
 <script setup>
-import { ref, computed, onMounted } from 'vue'
+import { ref, computed, onMounted, reactive } from 'vue'
 import { api, API_ENDPOINTS } from '../../config'
 
 const verified = ref(false)