docs: improve README demo video quality

[Guo-Yixin]

Summary

• Replace the 720x405 animated README preview with a sharp 1920x1080 poster linked to the full demo.
• Re-encode the full 89-second MP4 at high quality while preserving 1920x1080, 30 fps, and duration.
• Update English and Chinese README copy to identify the 1080p demo.

Verification

• Confirmed video metadata and decoded the entire MP4 successfully.
• Confirmed the poster is a 1920x1080 PNG.
• git diff --check passed.