Update project page with hand-control demo

[ryanontheinside]

Summary

• Replace the prompt-blend sample card with the new bird hand-control demo video from R2
• Reframe the card copy around DEMON supporting custom MediaPipe/performance interfaces
• Add a poster still generated from the new video

Checks

• Served docs locally at http://127.0.0.1:8080/ and confirmed the new video URL/copy are present and the old prompt-blend sample URL/title are absent
• Verified the generated poster image visually