feat: wire WebRTC call signaling end-to-end

Connects all call infrastructure so calls flow through the system:

- EventProcessor routes unwrapped call events (offer/answer/ICE/
  hangup/reject) from gift wraps to CallManager
- CallController orchestrates WebRTC session lifecycle: creates
  PeerConnection, generates SDP offers/answers, exchanges ICE
  candidates via gift-wrapped events, and manages foreground service
- AccountViewModel initializes CallManager + CallController and
  wires answer/ICE callbacks between them
- Account.publishCallSignaling() publishes gift-wrapped events
- DM chat top bar gets a call button (1-on-1 rooms only) that
  initiates a voice call and navigates to ActiveCall screen
- ActiveCall route registered in AppNavigation with CallScreen

https://claude.ai/code/session_017hZm7yu7CzmcQgZGSaqSXS

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

@@ -51,6 +51,9 @@ class CallManager(
     private val _state = MutableStateFlow<CallState>(CallState.Idle)
     val state: StateFlow<CallState> = _state.asStateFlow()
 
+    var onAnswerReceived: ((CallAnswerEvent) -> Unit)? = null
+    var onIceCandidateReceived: ((CallIceCandidateEvent) -> Unit)? = null
+
     private var timeoutJob: Job? = null
 
     companion object {
@@ -115,6 +118,7 @@ class CallManager(
 
         _state.value = CallState.Connecting(current.callId, current.peerPubKey, current.callType)
         cancelTimeout()
+        onAnswerReceived?.invoke(event)
     }
 
     fun onCallRejected(event: CallRejectEvent) {
@@ -127,8 +131,7 @@ class CallManager(
     }
 
     fun onIceCandidate(event: CallIceCandidateEvent) {
-        // ICE candidates are handled by the WebRTC session directly.
-        // This method exists for the call manager to validate the call-id.
+        onIceCandidateReceived?.invoke(event)
     }
 
     fun onPeerConnected() {