feat: add Copy Result button with clipboard support

[Ai-chan-0411]

Summary

• Adds a "Copy Result" button at the bottom of the result dashboard
• Copies comparison summary (winner, scores) as formatted JSON to clipboard
• Shows a success indicator (checkmark + "Copied!") for 2 seconds after copying
• Uses navigator.clipboard API with lucide-react Copy/Check icons

Closes #35

[vercel]

@aoi-dev-0411 is attempting to deploy a commit to the osama's projects Team on Vercel.

A member of the Team first needs to authorize it.

[github-actions]

Thank you for the pull request! ✅

A maintainer will review this soon. Please be patient while we take a look. 🙌

[Ai-chan-0411]

Closing this PR as there has been no maintainer feedback for several weeks. Thank you for the opportunity to contribute! Feel free to reopen if you would like these changes.