feat: wire Activity Feed to real API data (#822)

[aglichandrap]

Summary

Wires the home page Activity Feed to fetch real events from GET /api/activity instead of showing hardcoded mock data.

Resolves #822

Changes

• frontend/src/api/activity.ts — New API client for /api/activity endpoint with graceful fallback
• frontend/src/hooks/useActivity.ts — React Query hook with 30-second auto-polling
• frontend/src/components/home/ActivityFeed.tsx — Imports shared ActivityEvent type from API module
• frontend/src/pages/HomePage.tsx — Wires useActivity() hook, passes real events to <ActivityFeed />

Acceptance Criteria

• Activity feed shows real events from the API
• Events update without page refresh (30s polling via refetchInterval)
• Shows mock data as fallback when API is unavailable
• Graceful error handling

Technical Notes

• Uses the same apiClient pattern as bounties.ts and leaderboard.ts
• React Query placeholderData: [] prevents loading flash
• retry: false avoids noisy retries when the endpoint doesn't exist yet
• Mock data preserved as fallback until the backend endpoint is deployed