docs(readme): rebuild the demo GIF from the dark-theme master

Same recipe as before — 720x406, 12 fps, 64 colours, palettegen with
stats_mode=diff and a bayer dither — against a new master: OpenScreen in its
dark theme, editing a browser session rather than the old dashboard.

Smaller than what it replaces, 5,137,376 against 7,615,727 bytes, because the
dark UI and the flatter content quantize better.

The alt text still describes it exactly, so it is unchanged.

Author: sepion02