.empty-canvas-hint existed in editor.css but was never rendered
anywhere. Wire it up in Canvas.tsx: an EmptyCanvasHint component reads
Craft's ROOT node via useEditor and shows the hint once ROOT exists
with zero children, hiding again the instant something is dropped in
or while a drag is in progress. It's absolutely positioned over the
Frame with pointer-events: none so it never intercepts clicks/drops
meant for the underlying (empty) canvas -- scoped to regular page
editing only, not the header/footer editing mode.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
