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

[liqiuniu]

Summary

Adds a real-time countdown timer component for bounty deadlines. Addresses Bounty #826.

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Components Added

` (new file)

Component	Description
`	Full countdown display with days/hours/minutes/seconds
`	Compact variant for bounty cards

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Features

• Real-time updates 鈥?Refreshes every second without page reload
• Visual urgency indicators:
  • Normal: Gray text
  • Warning (< 24h): Amber/yellow text
  • Urgent (< 1h): Red text with pulse animation
  • Expired: Shows "Expired" label
• Two display modes:
  • Full: 2d 05h 30m 15s (for detail page)
  • Compact: 2d 5h or 5h 30m or <1h (for cards)

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Pages Updated

Component	Change
`	Uses ` for deadline display
`	Uses ` in sidebar info

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Acceptance Criteria

• Timer displays on bounty cards and detail page
• Updates without page refresh (every second)
• Visual urgency indicators (warning < 24h, urgent < 1h)

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Testing

1. View any bounty with a deadline
2. Observe countdown updating in real-time
3. Check color changes as deadline approaches

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Related

Bounty #826 鈥?#826