Mobile Overhaul: Tab layout

[tpachmann]

Reason (Why?)
In the current mobile version of the openArtBrowser, the labels for the tabs are aligned vertically at full width.
We should investigate how to display them in a more compact way, because in some places they take up to half of the screen.
Additionally, the selected tab indication via the background is looking weird without the border at the bottom.
To showcase a few occurences, here are several screenshots from the mobile frontend:

Artwork Page	Artist Page	Movement Page

Solution (What?)

• Select Box: Display the tab labels as a select box.

Acceptance criteria
- as described in the solution -