fix: restore artboard creation and drag on canvas

Canvas.tsx: removed the `e.target === e.currentTarget` guard on the
artboard-creation click handler. The canvas has a full-size
position:absolute transform layer that intercepts all events, making
`e.target` never equal to `e.currentTarget`. Artboard creation was
silently broken for every click.

Artboard.tsx: added `onMouseDown` stopPropagation to the artboard root
div so clicks on an existing artboard don't bubble up and trigger the
creation path now that the guard is gone.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/app/src/components/canvas/Artboard.tsx

@@ -165,6 +165,7 @@ export function Artboard({ id, label, x, y, width, height, renderUrl }: Artboard
   return (
     <div
       style={{ position: 'absolute', top: effectiveY, left: effectiveX }}
+      onMouseDown={(e) => e.stopPropagation()}
       onClick={(e) => { e.stopPropagation(); selectArtboard(id); }}
     >
       {/* Label / drag handle */}