feat: add bounty countdown timer component (Bounty #826)

[zp6]

Bounty Countdown Timer Component

Closes #826

Implementation

• Shows days, hours, minutes, seconds remaining
• Updates in real-time (every second)
• Color changes: green (normal) → yellow (< 24h warning) → red + pulse (< 1h urgent)
• Shows "Expired" when deadline passes
• Compact mode for bounty cards, full mode for detail page

Usage

// Full mode
<BountyCountdown deadline={bounty.deadline} />

// Compact (for cards)
<BountyCountdown deadline={bounty.deadline} compact />

Acceptance Criteria

• Timer displays on bounty cards and detail page
• Updates without page refresh
• Visual urgency indicators (color + animation)

Wallet: zp6