Add lead/follow performance mode

Add a mode like lead/follow where a leader can pick a song, assign a key, and mirror/send that state to following devices.

As a band leader or music director, I can start a lead session, pick a song, and assign the performance key so connected follower devices move to the same song and key without each player manually searching or transposing.

As a follower, I can join a session and see the leader's current song and key with clear status, while retaining enough local control to avoid being stuck if the session or network fails.