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

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/call/CallManager.kt

@@ -110,7 +110,15 @@ class CallManager(
 
         if (!isFollowing(callerPubKey)) return
 
-        if (_state.value !is CallState.Idle) return
+        if (_state.value !is CallState.Idle) {
+            // Already in a call — send a "busy" reject so the caller gets
+            // immediate feedback instead of waiting for the 60s timeout.
+            scope.launch {
+                val result = factory.createReject(callerPubKey, callId, "busy", signer = signer)
+                publishEvent(result.wrap)
+            }
+            return
+        }
 
         val groupMembers = event.groupMembers()
 
@@ -341,12 +349,12 @@ class CallManager(
             }
 
             is CallState.Connecting -> {
-                peerPubKeys = current.peerPubKeys
+                peerPubKeys = current.peerPubKeys + current.pendingPeerPubKeys
                 callId = current.callId
             }
 
             is CallState.Connected -> {
-                peerPubKeys = current.peerPubKeys
+                peerPubKeys = current.allPeerPubKeys
                 callId = current.callId
             }