The mobile web interface is currently a far better way to use Proton Mail than the app. The UI looks better, the archive option is there when you view the message and it's better laid out. There seems to be no advantage whatsoever to using the app, which has an inferior UI and doesn't work offline. At the very least bring the UI of the app up to the quality of the mobile web interface, and make it work offline, or you may as well not have a mobile app.