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>
