Add useWebSocket composable with:
- Auto-connect on login, disconnect on logout
- Exponential backoff with ±25% jitter for reconnects
- Auth failure detection to prevent reconnect loops
- Trailing slash stripping from API_URL
- Overlapping reconnect prevention
- visibilityState check for fallback polling
Replace notification polling with real-time WebSocket push in the
Notifications component. Initial state is still loaded via REST on
mount, with fallback polling when WebSocket is disconnected. Incoming
notifications are deduplicated against already-loaded REST data.
Notifications are reloaded via REST on WS disconnect to catch missed
events.
kolaente