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/.github/actions/setup-node/action.yml

@@ -0,0 +1,40 @@
+name: 'Setup Node'
+
+description: 'Setup node and pnpm'
+
+runs:
+  using: 'composite'
+  steps:
+    - name: Install pnpm
+      uses: pnpm/action-setup@v4
+
+    - name: Install Node.js
+      uses: actions/setup-node@v4
+      with:
+        node-version-file: .node-version
+        cache: 'pnpm'
+
+    - name: Get pnpm store directory
+      shell: bash
+      run: |
+        echo "STORE_PATH=$(pnpm store path --silent)" >> $GITHUB_ENV
+
+    - uses: actions/cache@v4
+      name: Setup pnpm cache
+      if: ${{ github.ref_name == 'main' }}
+      with:
+        path: ${{ env.STORE_PATH }}
+        key: ${{ runner.os }}-pnpm-store-${{ hashFiles('**/pnpm-lock.yaml') }}
+        restore-keys: |
+          ${{ runner.os }}-pnpm-store-
+
+    - uses: actions/cache/restore@v4
+      if: ${{ github.ref_name != 'main' }}
+      with:
+        path: ${{ env.STORE_PATH }}
+        key: |
+          ${{ runner.os }}-pnpm-store-
+
+    - name: Install dependencies
+      shell: bash
+      run: pnpm install --frozen-lockfile