core: implement real-time pool stats with Supabase Realtime

[EmeditWeb]

Problem

The sponsor dashboard shows static pool stats that
only update on page reload. Sponsors need live updates.

What To Build

1. Create useRealtimePool hook
2. Create useRealtimeLoan hook
3. Connection status indicator in Navbar
4. Offline handling

Files To Touch

• src/hooks/useRealtimePool.ts (new)
• src/hooks/useRealtimeLoan.ts (new)
• src/components/layout/Navbar.tsx
• src/pages/Sponsors.tsx

Acceptance Criteria

• Pool stats update without page reload
• Visual indicator on data change
• Connection status shown in Navbar
• Offline state handled gracefully
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue