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-proto/src/packet.rs

@@ -156,6 +156,14 @@ impl MediaHeader {
     }
 }
 
+/// A user visible in the signal presence list.
+#[derive(Clone, Debug, Serialize, Deserialize)]
+pub struct PresenceUser {
+    pub fingerprint: String,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub alias: Option<String>,
+}
+
 /// Quality report appended to a media packet when Q flag is set (4 bytes).
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub struct QualityReport {
@@ -1020,6 +1028,16 @@ pub enum SignalMessage {
         reason: Option<String>,
     },
 
+    // ── Signal presence ───────────────────────────────────────────
+
+    /// Relay broadcasts the list of currently registered signal
+    /// users to all connected clients. Sent on every register/
+    /// deregister so clients can maintain a live lobby user list.
+    PresenceList {
+        /// List of online users. Each entry is { fingerprint, alias }.
+        users: Vec<PresenceUser>,
+    },
+
     // ── Quality upgrade negotiation (#28, #29) ──────────────────
 
     /// Peer proposes upgrading to a higher quality profile.