Add PWA web manifest for standalone home screen installation (iOS/Android)

Currently copyparty has no `manifest.json`, so adding it to the home screen on iOS or Android just creates a plain bookmark that opens in the default browser. Adding a minimal web app manifest with `display: standalone` would make it installable as a proper PWA — opening without browser chrome, in its own window.

Minimum required: a `manifest.json` served at a well-known path, linked from the HTML head with ``, containing at minimum `name`, `start_url`, `display: standalone`, and an icon.

This is different from #37 (Web Share API) — that's about sharing files *to* copyparty; this is about the browsing UI itself being installable.

P.S. Thank you very much for such a great thing!

找出 copyparty 代码库中生成 HTML head 的位置，可能是在模板或提供 frontend 的 Python 模块中。创建一个包含所需字段（name、start_url、display: standalone、icon）的 manifest.json 文件，并将其放入静态 assets 目录。在 HTML head 中使用 将其链接起来。通过提供 app 并检查 manifest 是否可访问且链接正确来进行测试，然后在移动设备上或使用浏览器开发者工具验证其可安装性。

由索引模型根据 Issue 内容生成。