feat: Make floor plan upload sidebar closable

[resah]

In the lower left corner is a link to upload a new floor plan background image. This opens a sidebar on the right side, which will currently only close, once an image was uploaded and stored.

Add a close icon like in marker info side bar (when selecting a marker).

Hints:

• Change needed in https://github.com/AOEpeople/desk-compass/blob/main/workspaces/frontend/src/components/FloorPlanPaneSideBar.svelte
• Close button in marker info side bar: https://github.com/AOEpeople/desk-compass/blob/main/workspaces/frontend/src/components/InfoPaneSideBar.svelte#L149