adjust oversized scroll area for display arrangement

[FreddyFunk]

The bounding box for the scroll area in the display arrangement configuration page was way to wide, which looks kind of broken in most situations. This properly positions and resizes the widths.

Should fix #817 as well.

before:

2025-11-05.22-07-16.mp4

after:

2025-11-05.22-06-51.mp4

[jacobgkau]

Before this change, when dragging a screen past the edge of the area, it was contained within the area:

before.mp4

After this change, the display sticks out of the area all the way to the edge of the window:

after.mp4

Can we make it cut off at the edge of the area on the sides like it does on the top and bottom, and/or make the area start scrolling when dragging a display past the edge to avoid this problem? (In the above "after" video, I can see a horizontal scrollbar appear for a split second after dropping the display past the right side.)

[FreddyFunk]

This issue appears to be that as soon as the iced scrollable area has no scrollbar (e.g. does not need to be scrollable), the underlying custom draw function gets called at a different time, effectively changing the z-order, so that the custom draw function gets called later. This results in the overlapping of the "displays" with other UI elements. Alternatively one could rewrite the custom draw function, but this would be a bigger task, since zooming, padding for comfortable re-arranging of displays and moving focus would need to be thought of.

I will look for another task and mark this PR as draft. If anyone would like to pick up this PR, feel free to do so.