Fix the size of the layer scaling & dragging overlay UI

[dabbott]

While zoomed in:

• When scaling a layer, the UI that shows the dimensions, e.g. "30x30", is way too big
• When dragging a layer, the UI that shows "snap guides", e.g. the orange/purple lines and number indicating distance between layers, is way too big

I think both of these are in SnapGuides.tsx, and they mainly need to use const zoom = useZoom(); to get the current zoom.

BigDragUI.mov

BigDragUI2.mov