docs: regenerate README GIFs with the Xbot production character

The showcase GIFs (jumping-jacks hero + deadlift, squat, lateral-raise)
were still the old skin-toned mannequin. Re-ran `npm run gifs`, which
drives the live viewer headlessly and waits for characterActive, so the
GIFs now show the same Xbot figure (/models/xbot.glb) used in
production. Binary refresh only — README markup and text unchanged.

Author: a-baran-orhan