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/.changeset/config.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://unpkg.com/@changesets/config@3.0.0/schema.json",
+  "changelog": [
+    "@changesets/changelog-github",
+    { "repo": "vbenjs/vue-vben-admin" }
+  ],
+  "commit": false,
+  "fixed": [["@vben-core/*", "@vben/*"]],
+  "snapshot": {
+    "prereleaseTemplate": "{tag}-{datetime}"
+  },
+  "privatePackages": { "version": true, "tag": true },
+  "linked": [],
+  "access": "public",
+  "baseBranch": "main",
+  "updateInternalDependencies": "patch",
+  "ignore": []
+}