feat: Add dynamic Bounty Countdown Timer (T1 Bounty) (#826)

[messiawrq-spec]

Claims and resolves #826 (T1 Bounty).

• Implemented BountyCountdown component using React useState and setInterval to tick down every minute without full-page reloads.
• Dynamically shifts color thresholds to warn users when under 24 hours (amber) and under 1 hour (rose/red).
• Smoothly displays Expired once the deadline is surpassed.
• Replaces the static timeLeft output across BountyCard and BountyDetail with this new real-time component.

[nagiexplorer88]

This still misses the real-time part of #826 in the actual rendered UI. The PR adds frontend/src/components/bounty/BountyCountdown.tsx, but neither BountyCard.tsx nor BountyDetail.tsx imports or renders that component. Both screens still render the plain helper result:

{timeLeft(bounty.deadline)}

That value is only recomputed when the parent component renders, so the countdown will not tick down on its own without a page refresh or unrelated state update. The visible card/detail UI also does not use the <24h / <1h color thresholds from BountyCountdown; BountyDetail keeps a fixed text-status-warning, and the card keeps text-text-muted.

To satisfy the acceptance criteria, the displayed deadline in both BountyCard and BountyDetail should be replaced with the ticking countdown component (or equivalent stateful logic) so it updates without page refresh and shows the urgency colors in the actual UI.