feat: Add real-time bounty countdown timer with urgency indicators (Closes #826)

[sepulchralvoid666]

What this does

Adds a real-time countdown timer component for bounty deadlines with visual urgency indicators.

New Component: BountyCountdown

• Two variants: card (compact) and detail (full display)
• Real-time updates: Refreshes every second via useEffect + setInterval
• Color-coded urgency:
  • Normal: muted text with static dot
  • Warning (< 24h): yellow text with yellow dot
  • Urgent (< 1h): red text with pulsing red dot + URGENT badge
  • Expired: red Expired label
• Card variant: Compact 2d 5h 30m format with status dot
• Detail variant: Full 05:30:15 countdown with d/h/m/s labels and urgency badges

Integration

• BountyCard: Replaces Clock icon + timeLeft() with BountyCountdown variant="card"
• BountyDetail: Replaces static deadline display with BountyCountdown variant="detail"

How to test

1. Set a bounty deadline to < 1 hour from now — verify red pulsing dot + URGENT badge
2. Set deadline to < 24 hours — verify yellow warning styling
3. Set deadline to > 24 hours — verify normal muted styling
4. Set deadline to past — verify Expired label
5. Verify timer updates every second without page refresh

Closes #826

Wallet: 2JHa4QQWNV7AGSGsVeX1cwjZtFcmTTomb1TrJmQthoh3