feat: add real-time polling for group detail page

[nifanpinc]

Summary

Implements real-time polling for the group detail page, fulfilling all acceptance criteria from #35.

Changes

New: src/hooks/useGroupPolling.ts

• Custom React hook that polls the contract every 10 seconds
• Auto-starts polling when group status is Active or Forming
• Auto-stops when group is Completed or Failed
• Graceful error handling: keeps cached data on network failures
• Exposes refresh() for manual data refresh

Modified: src/app/groups/[id]/page.tsx

• Replaced static mock data with live contract data via useGroupPolling
• Added loading skeleton state during initial fetch
• Added error state with retry button
• Added inline warning banner when using cached data during errors
• Added manual Refresh Now button in actions panel

Modified: src/components/RoundProgress.tsx

• Added animated green Live indicator when polling is active
• Added lastUpdated timestamp display
• Progress bar turns green when all contributions are received
• Added payout ready message when round is complete
• Smooth 500ms transition animations on progress bar changes

Acceptance Criteria

• Poll contract for round status changes every 10 seconds
• Update member contribution status without page reload
• Show live progress bar of current round contributions
• Stop polling when round is complete

Closes #35

[nifanpinc]

Code Review Summary

Status: ??Approved

Excellent custom hook design (useGroupPolling). Ready to merge!