Web app mobile usability improvements

[lemon24]

The web app started with a plan of sorts, but with new features, more and more controls were added, cluttering the "UI" (below).

This is especially bad on mobile, where "show on hover" controls work poorly or not at all (e.g. buttons with input require a drag-then-tap-the-input-box which is hard to pull off).

Doing one or more of the following would greatly improve the experience:

• group controls in a sane manner; put them on separate lines where it makes sense
• don't hide controls if the device does not support hover
• hide controls that aren't used often in a <details> or similar

[lemon24]

Closing in favor of https://github.com/lemon24/reader/issues/318 (re-design wishlist).