fix(ci): 修复 frontend-security job 中的 pnpm 安装顺序问题

**问题描述：**
GitHub Actions 在 frontend-security job 中报错：
"Error: Unable to locate executable file: pnpm"

**根本原因：**
setup-node@v4 在尝试使用 pnpm cache 时，pnpm 还未安装

**解决方案：**
1. 调整步骤顺序：先安装 pnpm，再设置 Node.js
2. 升级 pnpm/action-setup 从 v2 到 v4
3. 明确指定 pnpm version: 9

**修改内容：**
- 将 "Set up pnpm" 步骤移到 "Set up Node.js" 之前
- 更新 pnpm/action-setup@v2 → pnpm/action-setup@v4
- 添加 version: 9 配置

**正确的步骤顺序：**
1. Checkout 代码
2. Set up pnpm (指定版本)
3. Set up Node.js (可以使用 pnpm cache)
4. Install dependencies

相关 Issue: #174

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.github/workflows/security-scan.yml

@@ -38,14 +38,16 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
+      - name: Set up pnpm
+        uses: pnpm/action-setup@v4
+        with:
+          version: 9
       - name: Set up Node.js
         uses: actions/setup-node@v4
         with:
           node-version: '20'
           cache: 'pnpm'
           cache-dependency-path: frontend/pnpm-lock.yaml
-      - name: Set up pnpm
-        uses: pnpm/action-setup@v2
       - name: Install dependencies
         working-directory: frontend
         run: pnpm install --frozen-lockfile