fix: correct group call hangup signaling and connection resilience

- Include pending peers in hangup notification so ringing users get
  immediate feedback instead of waiting for the 60s timeout
- Sign individual hangup events per peer with correct p-tag instead
  of reusing a single event with the first peer's tag
- Stop treating ICE DISCONNECTED as terminal; only hang up on FAILED
  since DISCONNECTED is often transient (network switch, packet loss)
- Send "busy" reject when receiving a call while already in one

https://claude.ai/code/session_01HpowdWMK77pA35ABn9XWpD

quartz/src/commonMain/kotlin/com/vitorpamplona/quartz/nipACWebRtcCalls/WebRtcCallFactory.kt

@@ -157,13 +157,17 @@ class WebRtcCallFactory {
         reason: String = "",
         signer: NostrSigner,
     ): GroupResult {
-        val template = CallHangupEvent.build(peerPubKeys.first(), callId, reason)
-        val signed = signer.sign(template)
+        // Each peer gets its own signed hangup with the correct `p` tag
+        // targeting that specific recipient.
+        var firstSigned: Event? = null
         val wraps =
             peerPubKeys.map { pubKey ->
+                val template = CallHangupEvent.build(pubKey, callId, reason)
+                val signed = signer.sign(template)
+                if (firstSigned == null) firstSigned = signed
                 GiftWrapEvent.create(event = signed, recipientPubKey = pubKey, expirationDelta = WRAP_EXPIRATION_SECONDS)
             }
-        return GroupResult(signed, wraps)
+        return GroupResult(firstSigned!!, wraps)
     }
 
     /**