Implement real-time presence indicators on kanban board

Description

Show live cursors and avatar presence for team members currently viewing the same board. Implement via WebSocket broadcast with graceful degradation to polling.

Implementation should follow the existing WebSocket connection pool pattern established in VLC-192. Use presence channels with heartbeat intervals of 5s and a 15s timeout before marking a user offline.

Sub-tasks 3

• VLC-242WebSocket presence channel adapter
• VLC-243Presence avatar cluster UI component
• VLC-244Offline detection + grace period logic