feat: add bounty countdown timer (Closes #826)

[liufang88789-ui]

Adds a reusable countdown timer component for bounty deadlines and integrates it into both bounty cards and the bounty detail sidebar.

Highlights:

• shows days / hours / minutes remaining
• updates automatically without page refresh
• warning color under 24 hours
• urgent color under 1 hour
• shows Expired after deadline passes
• used on bounty cards and detail page

Validation:

• npm test -- --run src/__tests__/bounty-countdown.test.tsx ✅
• npm run build still fails on existing upstream missing src/lib/animations and src/lib/utils imports unrelated to this change

Closes #826

Wallet: 7UqBdYyy9LG59Un6yzjAW8HPcTC4J63B9cZxBHWhhHsg

[liufang88789-ui]

Noticed the AI Code Review workflow failed with a GitHub API rate-limit error during trigger-review, rather than from a code-level review result. When rate limits recover, could you please rerun the review workflow for this PR? Thanks.