'circle' (default) or 'square'.
Both shapes occupy the same 29 px box — MapLibre's own control button size — so either one stacks
flush beneath the zoom buttons; the choice is purely how it reads. 'square' uses MapLibre's 4 px
control radius, so the collapsed widgets look like a continuation of its control group.
'circle' is the default, and keeps them legible as separate, optional tools.
Applies to every widget at once, immediately, including ones already collapsed.
Choose the shape of the icon button a collapsed widget shrinks to.