Investigate switching LANAPI discovery from broadcast to multicast

[tintinhamans]

LANAPI currently uses UDP broadcast for game discovery. This sends packets to every device on the subnet, regardless of whether they are running the game.

Multicast would limit traffic to only devices that have joined the relevant multicast group, reducing unnecessary network noise and potentially allowing discovery across subnet boundaries. It is worth investigating whether switching LANAPI to multicast would be a practical improvement over the current broadcast approach.