enhance lab calendar GUI

[A-Schmid]

Currently, the physical lab calendar's GUI is very cluttered and does not provide relevant information at the first glance.

There are several potential improvements:

1. Remove the unnecessary word "Reservierung" to save space. Ideally, one appointment should fit in one line.
   1b. Additional space could be saved by shortening the time string. For example, the "Uhr" could be replaced with a clock icon.
2. Add separation between current and upcoming appointments. The common Gestalt principles could help users differentiate between current and future appointments.
3. Add an indicator that shows whether a room is currently occupied. This could for example be a circle that is filled or empty depending on the room's occupation state.
4. Maybe remove rooms without current or upcoming appointments entirely?