Add real-time notifications for group events like new contributions, payouts, and disputes.
Acceptance Criteria:
- Set up notification provider with WebSocket or Server-Sent Events
- Show in-app notification bell with unread count
- Notification types: contribution received, payout distributed, dispute raised
- Notification dropdown with mark-as-read functionality
Files: new file src/components/NotificationBell.tsx, new file src/components/NotificationProvider.tsx, src/app/providers.tsx