Add bounty form validation errors

[nikkkkj]

Fixes #12.

Summary

• Add explicit form error state for title and reward validation
• Block empty titles and non-positive reward values before submission
• Render inline validation messages instead of alerts
• Clear field-specific errors when the user edits that field
• Add noValidate so the custom validation UI is consistently shown

Verification

• npm run lint
• npm run build
• Browser check: submitted an empty form and confirmed title/reward errors appear; typing a title clears the title error while preserving the reward error

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