feat: add countdown timer component for bounty deadlines (Fixes #826)

[zeroknowledge0x]

Summary

Implements a countdown timer component that shows time remaining until bounty deadline, as requested in issue #826.

Changes

• Created component with real-time countdown (updates every second)
• Shows days, hours, minutes, seconds remaining
• Color-coded urgency levels:
  • Normal: muted text color (>24h remaining)
  • Warning: amber color (<24h remaining)
  • Urgent: red color with pulse animation (<1h remaining)
  • Expired: grayed out when deadline passes
• Created with utility functions (timeLeft, timeAgo, formatCurrency, etc.)
• Updated to use CountdownTimer
• Updated to use CountdownTimer

Acceptance Criteria

• Timer displays on bounty cards and detail page
• Updates without page refresh (1-second interval)
• Visual urgency indicators (color changes and pulse animation)

Technical Details

• Uses React useEffect with setInterval for real-time updates
• Cleans up interval on unmount
• Stops updating when deadline expires
• Responsive design with Tailwind CSS

Closes #826

[zeroknowledge0x]

Hi maintainers! 👋

Just a friendly follow-up — all CI checks are passing:

• ✅ AI Code Review (trigger-review)
• ✅ Wallet Check (check-wallet)
• ✅ CodeRabbit review

The countdown timer component is ready for review. Happy to address any feedback!

Thanks for your time.