README: open with a video of a sketch running on Android

A nine-second clip replaces the hero screenshot in all three languages: a
habit tracker sketched in the editor on the left, and the app an AI coding
tool built from the generated prompt running on Android on the right. The
GIF plays inline on GitHub; the silent 720p mp4 is linked underneath.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>