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/.gitattributes

@@ -0,0 +1,11 @@
+# https://docs.github.com/cn/get-started/getting-started-with-git/configuring-git-to-handle-line-endings
+
+# Automatically normalize line endings (to LF) for all text-based files.
+* text=auto eol=lf
+
+# Declare files that will always have CRLF line endings on checkout.
+*.{cmd,[cC][mM][dD]} text eol=crlf
+*.{bat,[bB][aA][tT]} text eol=crlf
+
+# Denote all files that are truly binary and should not be modified.
+*.{ico,png,jpg,jpeg,gif,webp,svg,woff,woff2} binary