You could send a PR. It's reasonably straightforward to add a manifest to make the app installable (https://developer.mozilla.org/en-US/docs/Web/Progressive_web...), and to use Workbox to make it work offline since it's fully static (https://developer.chrome.com/docs/workbox).

The one caveat is that a PWA needs an icon but the project doesn't currently have one, so you'd have to design, find, or LLM-generate one.

You have to use "Add to home" menu item on Firefox for Android. But this web app doesn't seem to be a PWA.