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/rooms/panel/rooms_space_list.dart

@@ -95,7 +95,7 @@ class _SpaceRoomsListState extends ConsumerState<_SpaceRoomsList> {
       if (jr.isVoice) {
         // LiveKit only needs the room id + a display name, so connect even if the
         // matrix room object hasn't synced locally yet.
-        final call = ref.read(callStateProvider);
+        final call = ref.read(voiceChannelProvider);
         if (call.isActive) await call.hangUp();
         ref.read(activeVoiceRoomIdProvider.notifier).state = jr.roomId;
         await call.startCall(
@@ -199,7 +199,7 @@ class _SpaceRoomsListState extends ConsumerState<_SpaceRoomsList> {
     }
 
     void joinVoiceChannel(Room room) async {
-      final call = ref.read(callStateProvider);
+      final call = ref.read(voiceChannelProvider);
       final activeVoice = ref.read(activeVoiceRoomIdProvider);
       // Toggle: leave if already in this channel
       if (call.isVoiceChannel && activeVoice == room.id) {