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/packages/@core/ui-kit/shadcn-ui/components.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "https://shadcn-vue.com/schema.json",
+  "style": "new-york",
+  "typescript": true,
+  "tailwind": {
+    "config": "tailwind.config.mjs",
+    "css": "src/assets/index.css",
+    "baseColor": "slate",
+    "cssVariables": true
+  },
+  "framework": "vite",
+  "aliases": {
+    "components": "@vben-core/shadcn-ui/components",
+    "utils": "@vben-core/shared/utils"
+  }
+}