feat(frontend): Wire Activity Feed to Real API (#822)

[lui62233]

Fixes Bounty #822 - Wire Activity Feed to Real API Data

Changes

• useActivity.ts (new): React Query hook that fetches GET /api/activity
  • Refetches every 30 seconds (per acceptance criteria)
  • 5s timeout — non-critical path, fast fail is acceptable
  • Returns empty array on error → triggers MOCK_EVENTS fallback
  • retry: false for graceful degradation
• ActivityFeed.tsx: Export ActivityEvent type for reuse by the hook
• HomePage.tsx: Wire useActivity() hook → pass events prop to ActivityFeed

Acceptance Criteria

• Activity feed shows real events from the API
• Events update without page refresh (30s auto-refetch)
• Shows MOCK_EVENTS when API is unavailable (graceful fallback)

Notes

• If /api/activity does not yet exist in the backend, the component degrades to MOCK_EVENTS — consistent with existing demo-mode behavior
• Follow-up backend work (issue 🏭 Bounty T1: Wire Activity Feed to Real API Data #822 owner) can wire the actual endpoint without changing the frontend contract