feat: add live bounty countdown timer

[anansutiawan]

Summary

• Adds a reusable BountyCountdown component for bounty deadlines
• Shows days/hours/minutes remaining, with warning and urgent visual states
• Replaces static deadline text on bounty cards and detail pages with the live countdown
• Adds unit coverage for formatting, expired/invalid deadlines, urgency states, and timer updates

Test Plan

• npx vitest run src/components/bounty/BountyCountdown.test.tsx
• npx tsc --noEmit --jsx react-jsx --esModuleInterop --skipLibCheck src/components/bounty/BountyCountdown.tsx

Note: full npm run build is currently blocked on main by missing shared modules (src/lib/animations, src/lib/utils) unrelated to this change.

Closes #826

Wallet: 3LVUkJgwJXtaBpWS1NuAfMjV8xF1e8aNVg5kKg4YtD6s