Write or adopt a set of mobile friendly core apps

[roscale]

File manager, notes app, calendar, calculator, camera, etc. are all applications that should exist. If you install Phosh
or Plasma Mobile, you already have some preinstalled apps. Maybe the easiest would be to adopt already existing apps,
but we would lose design consistency. If we go this route it would be best if we can find some Flatpak apps that
integrate well.

A store app for installing Flatpak apps from Flathub without elevated privileges would be nice to have. Once again,
remains to be seen if we should adopt an already existing app or if we should write our own.

If time allows, I could write some of these apps in Flutter. Ideally, we would host our own Flatpak repository with our
own apps known to work well on a mobile device.