fix(signal): send PresenceList directly to new client after ack

The broadcast alone wasn't reaching the first client because its
recv loop hadn't started yet when the second client registered.
Now the relay sends PresenceList directly to the new client (right
after RegisterPresenceAck) AND broadcasts to all others.

This guarantees every client gets the full user list:
- New client: via direct send (queued before recv loop starts)
- Existing clients: via broadcast

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

crates/wzp-relay/src/main.rs

@@ -1016,10 +1016,16 @@ async fn main() -> anyhow::Result<()> {
 
                 info!(%addr, fingerprint = %client_fp, alias = ?client_alias, "signal client registered");
 
-                // Broadcast updated presence to all signal clients
+                // Send the full presence list directly to the new
+                // client (guaranteed delivery — their recv loop is
+                // about to start). Then broadcast to all OTHER
+                // clients so they learn about the new user.
                 {
                     let hub = signal_hub.lock().await;
                     let presence = hub.presence_list();
+                    // Direct send to new client (arrives right after ack)
+                    let _ = transport.send_signal(&presence).await;
+                    // Broadcast to everyone else
                     hub.broadcast(&presence).await;
                 }