- Fullscreen editor: toolbar and Save/Cancel now float above the canvas,
  which takes the entire window (1344x896 vs 1147x765 at 1440x900).
- Extension composer widened to 1100px, drawing area raised to 62vh.
- Text tool renders ~2.4x larger (strokeWidth * 12, min 24px); size
  slider now goes up to 20.
- Screenshots never contain the plugin's own UI: the background hides the
  overlay host around every captureVisibleTab (covers region capture,
  full-page tiles and recorder step screenshots); the region selector
  keeps it hidden for the whole stitch via a refcounted module.
- e2e saves a step screenshot for inspection; fixed its file download to
  resolve attachment id -> file id.

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