feat: improve mobile responsive polish (Closes #824)

[liufang88789-ui]

Polishes the mobile experience around the main responsive pain points called out in the issue: bounty cards, navbar/mobile navigation, hero readability, and footer overflow behavior.

Highlights:

• bounty cards now wrap and stack metadata more cleanly on small screens
• mobile navigation/sign-in flow is more robust and less cramped
• hero terminal + CTA layout is easier to read at 375px and 768px widths
• footer token/social sections avoid overflow and preserve truncation behavior
• no horizontal-scroll-prone long text blocks in these key surfaces

Validation:

• npm test -- --run src/__tests__/responsive-polish.test.tsx ✅

Note:

• this also includes the lightweight BountyCountdown component because the current BountyCard uses it and the branch needs to remain self-contained.

Closes #824

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.