Preview resize handles

Adds drag handles to both edges of the preview so you can resize it around the centre, which is handy for container-query work. The width readout updates as you drag and the matching breakpoint lights up as you cross it; past your widest breakpoint you can keep dragging to preview any width up to the full canvas. Experimental: above your widest breakpoint the builder has no canvas size of its own, so the handle sizes the canvas itself — a Builderius update could affect that.