Missing a Manifest file for installability (PWA support)

When visiting web.localsend.org on a browser supporting PWAs (Chrome, Opera, Edge, etc.), installation could be offered if a [Web Manifest was provided](https://developer.mozilla.org/en-US/docs/Web/Progressive_web_apps/Manifest)

Happy to provide a PR if the project is open to it.

Refs: https://github.com/localsend/localsend/issues/2727