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-proto/src/packet.rs

@@ -686,6 +686,10 @@ pub struct RoomParticipant {
     pub fingerprint: String,
     /// Optional display name set by the client.
     pub alias: Option<String>,
+    /// Relay label — identifies which relay this participant is connected to.
+    /// None for local participants, Some("Relay B") for federated.
+    #[serde(default)]
+    pub relay_label: Option<String>,
 }
 
 /// Reasons for ending a call.