- ScreenshotViewer: the fullscreen editor stage now centers the editor
  block (it sized to the canvas and hugged the left edge on wide windows)
- overlay App: unmount the picker (highlight + tooltip) as soon as an
  element is picked, before the pristine capture happens
- RegionSelector: hide the overlay locally around the pristine capture
  too, so it no longer depends on the background's message roundtrip

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