Add image upload, work phase tabs & background image

[LBBO]

Closes #138, closes #134, closes #137, closes SIO-65

Reviewer's task:

• [x] I didn't see no code smells
• [x] The live preview works as expected -> no regressions found

[render[bot]]

Your Render PR Server URL is https://screenario-pr-147.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-clphkh146foc73cgfq50.

[liam-k]

Looking good! Some UX/styling suggestions resulting from this change though:

• [x] when a background image is set, the transparent buttons are hard to see. we should probably give them a white background
• [x] we should add another button (not sure if in the bottom bar where everything is, or in the WorkPhase component) which just removes the current background image in one click. Otherwise users have to open WorkPhase, look for the currently set background image, and open its menu again... probably annoying
• [x] if an uploaded custom image is taller than wide, there are some scrolling issues (in the workphase window)

[LBBO]

The scrolling issues seem to be independent from these changes. Should I still look into them?

[LBBO]

And what about adding a hamburger menu in the top bar? i find the bottom bar is getting a bit crowded. another idea for adding the clear button would be just somewhere more permanent in the workphase window

[liam-k]

The scrolling issues seem to be independent from these changes. Should I still look into them? hmm, u sure? just to be clear, this what i'm experiencing:

default images are scrollable (they don't need to be but whatever) while custom images behave just ... weird.

re: button placements, i’ll figure something out. i’m not quite happy with the bottom button row either

[LBBO]

• [x] context menu -> zwei buttons, siehe positionierung tool
• [x] unten buttons bei hover transparent
• [x] popup normal scrollbar
• [x] positionierung image upload fixen
• [x] button clear background