feat: relay_label in RoomParticipant + tagged remote participants

RoomParticipant.relay_label identifies which relay a participant is
connected to. Local participants have None, federated participants
get tagged with the peer relay's label when storing remote_participants.

This enables clients to group participants by relay in the UI.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/wzp-relay/src/room.rs

@@ -180,6 +180,7 @@ impl Room {
             .map(|p| wzp_proto::packet::RoomParticipant {
                 fingerprint: p.fingerprint.clone().unwrap_or_default(),
                 alias: p.alias.clone(),
+                relay_label: None, // local participant
             })
             .collect()
     }