Contribute README screenshots and demo GIF

[dushaobindoudou]

Background

README has 6 screenshot placeholders + 1 demo GIF placeholder. Real assets would 10× the project's professional appearance on GitHub.

What's needed

• 6 PNG screenshots at 1920×1200 (retina) saved to docs/assets/
• 1 demo GIF at 800px wide, <10MB

Acceptance

• All 6 screenshots committed under docs/assets/
• README image links updated
• README_CN.md image links updated

Full brief: https://github.com/dushaobindoudou/ovo/blob/main/docs/GOOD_FIRST_ISSUES.md (issue #6)

⏱ ~1 h · 🏷 documentation, design