eat: polish bounty card UI and align media handling with Next.js rules。

[omcdecor-cyber]

Summary

• BountyCard Overhaul: Refreshed the UI with a premium look, including gradient hover effects and difficulty badges.
• Robustness: Implemented progress clamping and currency formatting for safer props handling.
• Next.js Compliance: Replaced raw <img> tags with next/image in the leaderboard and updated next.config.mjs to allow remote domains, clearing all lint warnings.
• Tooling: Added clsx dependency for cleaner conditional class management.

Testing

• Ran npm run lint: Passed with zero warnings.
• Verified responsive layout and accessibility (ARIA roles added to progress bars).

Impact

Improves the professional visual standard of the hackathon kit and ensures the codebase follows Next.js best practices for media handling.