Back-to-top button on desktop

[graphieros]

This perhaps can be discussed.

1. The back-top-top is currently only visible on mobile.
   I think it would be fitting to also show it on desktop.

2. The default smooth scroll behavior could be normalized so that its timing be fixed, regardless of the scroll height. This would require JS, since no easing nor timing is available in the browser API.

[MatteoGabriele]

We should also take reduce-motion into account and, if present, proceed directly to the top without any transition.