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-client/src/featherchat.rs

@@ -138,6 +138,7 @@ pub fn signal_to_call_type(signal: &SignalMessage) -> CallSignalType {
         | SignalMessage::UpgradeResponse { .. }
         | SignalMessage::UpgradeConfirm { .. }
         | SignalMessage::QualityCapability { .. } => CallSignalType::Offer, // quality negotiation
+        SignalMessage::PresenceList { .. } => CallSignalType::Offer, // lobby presence
         SignalMessage::QualityDirective { .. } => CallSignalType::Offer, // relay-initiated
     }
 }