Fix a `<AuToggleSwitch>` shrinking issue

[Windvis]

This change prevents the visual switch from shrinking below its minimum width.

Before:

After:

Supersedes #377

[abeforgit]

Not to be pedantic, but elements can't shrink below their minimum width anyway (or at least not in a flex context). The toggle just doesn't set any min-width at all. My original solution was also wrong (cause I misremembered how width, min-width and max-width interact), but it seems weird to me to disable any and all shrinking rather than just setting a minimum width