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.