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/package.json

@@ -0,0 +1,21 @@
+{
+  "name": "@vben/backend-mock",
+  "version": "0.0.1",
+  "description": "",
+  "private": true,
+  "license": "MIT",
+  "author": "",
+  "scripts": {
+    "build": "nitro build",
+    "start": "nitro dev"
+  },
+  "dependencies": {
+    "@faker-js/faker": "catalog:",
+    "jsonwebtoken": "catalog:",
+    "nitropack": "catalog:"
+  },
+  "devDependencies": {
+    "@types/jsonwebtoken": "catalog:",
+    "h3": "catalog:"
+  }
+}