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/api/demo/bigint.ts

@@ -0,0 +1,32 @@
+import { eventHandler, setHeader } from 'h3';
+import { verifyAccessToken } from '~/utils/jwt-utils';
+import { unAuthorizedResponse } from '~/utils/response';
+
+export default eventHandler(async (event) => {
+  const userinfo = verifyAccessToken(event);
+  if (!userinfo) {
+    return unAuthorizedResponse(event);
+  }
+  const data = `
+  {
+    "code": 0,
+    "message": "success",
+    "data": [
+              {
+                "id": 123456789012345678901234567890123456789012345678901234567890,
+                "name": "John Doe",
+                "age": 30,
+                "email": "john-doe@demo.com"
+                },
+                {
+                "id": 987654321098765432109876543210987654321098765432109876543210,
+                "name": "Jane Smith",
+                "age": 25,
+                "email": "jane@demo.com"
+                }
+            ]
+  }
+  `;
+  setHeader(event, 'Content-Type', 'application/json');
+  return data;
+});