This seems wrong and was especially noticeable since transform snapping
does account for it (which was reported in T92494).
Now divide the DotGridLevelInfo step_factor by the default of 20 for
U.widget_unit and scale it later by the actual interface scale.

note: when zooming, this will still always snap to the smallest dot
level (not sure, with a bit more work it could be possible to only snap
to the lowest visible level after fading?)