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!