Add MCP health check loop, auto-reconnect, and system toast notifications ...

Backend:
- McpManager: keep all configured servers in the pool even if connect()
  fails at startup; health-check loop (30s) tries to reconnect dead servers
  and verifies live ones with list_tools()
- McpManager: set_on_server_connected callback re-registers tools when a
  dead server comes back online
- McpClient: add mark_disconnected() for silent drop detection
- McpStatusTool: skip list_tools() for disconnected servers
- Orchestrator: broadcast mcp_status_update to all WebSocket sessions
- New event type McpStatusUpdate with server_name, status, tool_count

Webclient:
- useWebSocket: handle mcp_status_update → toast.success/toast.error

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>