Add bounty countdown timer

[nikkkkj]

Fixes #826.

Summary

• Add a reusable CountdownTimer component for bounty deadlines
• Display days/hours/minutes remaining and update every minute without refresh
• Show warning styling under 24 hours and urgent styling under 1 hour
• Show Expired once the deadline has passed
• Render the timer on bounty cards and bounty detail pages
• Add focused tests for formatting, warning, urgent, and expired states
• Restore missing frontend lib/animations and lib/utils helpers so existing imports resolve during production build

Verification

• npm test -- countdown-timer.test.ts
• npm run build