feat: WebSocket auto-reconnect with backoff

[kagura-agent]

Problem

If the server restarts, the frontend stays disconnected until manual refresh.

Solution

Implement reconnect logic in useWebSocket hook with exponential backoff.

Acceptance Criteria

• Auto-reconnect on disconnect
• Exponential backoff (1s, 2s, 4s, max 30s)
• Visual indicator during reconnection
• Re-fetch room/agent list on reconnect