fix: 添加PDF打印机步骤到安装确认列表

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

setup_cups.sh

@@ -759,6 +759,8 @@ main() {
     echo "  $step. 将当前用户添加到 lpadmin 组"
     ((step++))
     echo "  $step. 启动并设置开机自启"
+    ((step++))
+    echo "  $step. 安装虚拟 PDF 打印机（测试用）"
     echo ""
     read -p "是否继续? [Y/n]: " confirm < /dev/tty
     confirm=${confirm:-Y}