PFE uses lit, so it makes sense to just use that for the main
application as well. Web components are pretty much React built into the
web platform, and lit adds some convenience around that.
This is mostly a demo -- for real Cockpit pages, PF Elements is still
missing too many components.
This needs the usual "I know the `app` element exists" annotation, plus
dealing with a non-existing /etc/hostname (in which case the watch will
return `null`, and we shouldn't poke that into a `string` state).
