chore: 初始化前端仓库并迁移 admin-ui

- 创建 rui-frontend 前端仓库
- 迁移 admin-ui 管理后台
- 创建 cashier-mobile 和 customer-mobile 占位项目
- 配置 pnpm workspace

cashier-mobile/package.json

@@ -0,0 +1,15 @@
+{
+  "name": "cashier-mobile",
+  "version": "0.1.0",
+  "description": "收银系统移动端",
+  "private": true,
+  "type": "module",
+  "scripts": {
+    "dev": "echo '待配置开发命令'",
+    "build": "echo '待配置构建命令'",
+    "lint": "echo '待配置'",
+    "type-check": "echo '待配置'"
+  },
+  "dependencies": {},
+  "devDependencies": {}
+}