feat: 添加完整的前端管理系统 (VbenAdmin)

- 添加基于 VbenAdmin + Vue3 + Element Plus 的前端管理系统
- 包含完整的 UI 组件库和工具链
- 支持多应用架构 (web-ele, backend-mock, playground)
- 包含完整的开发规范和配置
- 修复 admin 目录的子模块问题，确保正确提交

admin/apps/backend-mock/nitro.config.ts

@@ -0,0 +1,20 @@
+import errorHandler from './error';
+
+process.env.COMPATIBILITY_DATE = new Date().toISOString();
+export default defineNitroConfig({
+  devErrorHandler: errorHandler,
+  errorHandler: '~/error',
+  routeRules: {
+    '/api/**': {
+      cors: true,
+      headers: {
+        'Access-Control-Allow-Credentials': 'true',
+        'Access-Control-Allow-Headers':
+          'Accept, Authorization, Content-Length, Content-Type, If-Match, If-Modified-Since, If-None-Match, If-Unmodified-Since, X-CSRF-TOKEN, X-Requested-With',
+        'Access-Control-Allow-Methods': 'GET,HEAD,PUT,PATCH,POST,DELETE',
+        'Access-Control-Allow-Origin': '*',
+        'Access-Control-Expose-Headers': '*',
+      },
+    },
+  },
+});