Persist bounty filter state in URL

[nikkkkj]

Fixes #6.

Summary

• Hydrate bounty filters from difficulty and minReward URL query params
• Update the URL when filter controls change
• Keep default filters out of the URL for cleaner links
• Include the existing minimal form validation compile fix so production build is not blocked by the current errors reference

Verification

• npm run lint
• npm run build
• Browser check: opened /bugs/filter?difficulty=Hard&minReward=600, refreshed, and confirmed the Hard/600 filters and filtered count persisted

Note: lint/build still report the existing Next.js <img> warning in src/components/leaderboard.tsx, but the build completes successfully.