refactor: M2-Call-Pilot – Module call_signaling + voice_channel mit Fassaden (Entscheidung 2)

- CallSignalingService/callSignalingProvider und VoiceChannelService/
  voiceChannelProvider als einzige oeffentliche Schnittstellen
- voip_manager.dart / livekit_call_manager.dart in ihre Module verschoben,
  Logik unangetastet (nur Klassen-Koepfe, @override, Imports, Provider-Namen)
- Alle Konsumenten entkoppelt; Kompositions-Punkt matrix_client.dart als
  dokumentierte Ausnahme. analyze sauber, 29 Tests gruen, Windows-Start ok.
  Call-Klick-/Geraetetest steht aus (PROGRESS.md)

lib/features/chat/view/chat_header.dart

@@ -167,13 +167,13 @@ class _ChatHeader extends ConsumerWidget {
     if (room.isDirectChat) {
       // Use native Matrix VoIP for 1:1 DMs
       ref.read(viewModeProvider.notifier).state = ViewMode.voice;
-      await PyramidVoipManager.instance.startCall(room.id, video: cam);
+      await ref.read(callSignalingProvider).startCall(room.id, video: cam);
       return;
     }
 
     // Use LiveKit for group voice rooms
     ref.read(viewModeProvider.notifier).state = ViewMode.voice;
-    await ref.read(callStateProvider).startCall(
+    await ref.read(voiceChannelProvider).startCall(
       roomName: roomId,
       roomDisplayName: room.getLocalizedDisplayname(),
       identity: 'user_${DateTime.now().millisecondsSinceEpoch}',
@@ -576,7 +576,7 @@ class _DmExpandingHeaderState extends ConsumerState<_DmExpandingHeader> {
 
   void _startCall({required bool cam}) async {
     ref.read(viewModeProvider.notifier).state = ViewMode.voice;
-    await PyramidVoipManager.instance.startCall(widget.room.id, video: cam);
+    await ref.read(callSignalingProvider).startCall(widget.room.id, video: cam);
   }
 }