docs(threading): add Remotion simulated demo for #509

public/assets/threading-remotion-demo.gif shows the flat inbox (5 emails),
T toggle to threaded mode (3 rows with the (3) count on the JWZ-grouped
root), Enter to expand the thread tree (showing the original + 2 replies
indented with ↪ markers). Rendered by Remotion at 1280x720, encoded via
--scale=0.6 --every-nth-frame=3.

Author: mvanhorn