Wire home activity feed to API

[JustABiologist]

Summary

• Replaces the hardcoded home page activity feed with /api/activity data through a typed activity API client and React Query hook.
• Adds 30-second auto-refresh, empty-state handling, and an unavailable-state fallback when the API cannot be reached.
• Adds shared frontend utility/animation helpers that existing components already import, and narrows .gitignore so frontend/src/lib source files are tracked.

Verification

• npm test -- activity-api.test.ts home-activity-feed.test.tsx
• npm run build
• git diff --check

Note: full npm test still fails on existing unrelated test imports for missing Playwright/admin/tokenomics/disputes/private-backend modules; the new activity tests and production build pass.

Bounty

Addresses #822.

Payment request: Solana wallet BPX8AsqzNRmvFnTN5wGF8Zxsp5znwdZA5BEyEAWyhkCM. If this bounty supports PayPal/manual payout instead, PayPal stab.me.papi@gmail.com is also available.

[nagiexplorer88]

This still does not actually wire the feed to a working backend activity source in this repo. The PR adds a frontend client that always calls /api/activity, but I do not see any backend route or handler for /api/activity added here, and code search in the repository does not show an existing backend activity endpoint either.

With the current diff, ActivityFeed will request an endpoint that the app cannot serve, then fall into the unavailable state instead of showing real bounty completions, PR submissions, or payouts. To satisfy #822, this needs either the backend /api/activity route implemented/wired in this repo, or the frontend client pointed at an existing activity endpoint with the matching response shape.