Show empty-canvas hint on a page with no components yet

.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>

craft/src/styles/editor.css

@@ -1225,6 +1225,8 @@ body {
    Empty Canvas State
    -------------------------------------------------------------------------- */
 .empty-canvas-hint {
+  position: absolute;
+  inset: 0;
   display: flex;
   flex-direction: column;
   align-items: center;
@@ -1234,6 +1236,9 @@ body {
   font-size: 13px;
   text-align: center;
   gap: 12px;
+  /* Overlays the drop area without intercepting drags/clicks meant for the
+     underlying (empty) canvas root -- see Canvas.tsx. */
+  pointer-events: none;
 }
 
 .empty-canvas-hint i {