feat: add bounty countdown timer component (#826)

[CHY9213]

Bounty Countdown Timer Component

Creates a countdown timer that shows time remaining until bounty deadline.

Features

• Displays days, hours, minutes remaining
• Updates in real-time (every 60 seconds, no page refresh needed)
• Visual urgency indicators:
  • 🟢 Green (>24 hours remaining)
  • 🟡 Amber (<24 hours - warning)
  • 🔴 Red (<1 hour - urgent)
• Shows "Expired" when deadline passes
• Dark mode compatible (uses Tailwind dark: variants)
• Lightweight: pure React + Tailwind, no external dependencies

Usage

import { BountyCountdownTimer } from '../components/bounty/BountyCountdownTimer';

// In bounty card or detail page:
<BountyCountdownTimer deadline={bounty.deadline} />
<BountyCountdownTimer deadline={bounty.deadline} className=text-base />

Closes #826

Wallet: 2oWEBzKDQo9KYxAFYMPrR8QiwKJia4vbSXrbV4ep7eTz