Show feedback when copying leaderboard addresses

[pntech20]

Summary

• show a short Copied state after copying a leaderboard address
• keep the existing refresh and ranking logic unchanged
• style the feedback inline with the current leaderboard table

Closes #15

Testing

• npm run build
• git diff --check

Note: npm run lint is currently blocked because the repo does not include an ESLint v9 config file.

[vercel]

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