feat: 添加前端页面截图

包含登录、商品、购物车、订单、收藏、个人中心、商家概览、管理员审核等页面截图

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.gitignore

@@ -61,5 +61,7 @@ __pycache__/
 *.pyc
 *.pyo
 
-# Screenshots (root level)
+# Screenshots (root level only)
-*.png
+/*.png
+# Allow frontend screenshots
+!frontend/screenshots/