PWA: Improve manifest.json for more reliable installation prompts

[lastzero]

It seems that in some cases the Mobile App (PWA) is not reliably offered for installation, so this is an attempt to improve the situation by updating the manifest.json file.

Docs: https://developer.mozilla.org/en-US/docs/Web/Manifest

[lastzero]

It would be really helpful if someone who has experience with this could look at these changes.