feat(signal): PresenceList broadcast for lobby user discovery

New signal infrastructure for the lobby-first UI:

- PresenceUser struct: { fingerprint, alias }
- SignalMessage::PresenceList: relay broadcasts full user list
  to all signal clients on every register/deregister
- SignalHub::presence_list(): builds the list from connected clients
- SignalHub::broadcast(): sends to ALL signal clients
- Relay calls broadcast on register + unregister
- Desktop emits "presence_list" signal-event to JS frontend

This gives clients real-time visibility of who's online via the
signal channel, without needing to join a voice room first.

603 tests pass, 0 regressions.

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

crates/wzp-relay/src/main.rs

@@ -1016,6 +1016,13 @@ async fn main() -> anyhow::Result<()> {
 
                 info!(%addr, fingerprint = %client_fp, alias = ?client_alias, "signal client registered");
 
+                // Broadcast updated presence to all signal clients
+                {
+                    let hub = signal_hub.lock().await;
+                    let presence = hub.presence_list();
+                    hub.broadcast(&presence).await;
+                }
+
                 // Signal recv loop
                 loop {
                     match transport.recv_signal().await {
@@ -1560,6 +1567,9 @@ async fn main() -> anyhow::Result<()> {
                 {
                     let mut hub = signal_hub.lock().await;
                     hub.unregister(&client_fp);
+                    // Broadcast updated presence to remaining clients
+                    let presence_msg = hub.presence_list();
+                    hub.broadcast(&presence_msg).await;
                 }
                 {
                     let mut reg = presence.lock().await;