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/layout/app_shell.dart

@@ -9,9 +9,10 @@ import 'package:pretty_qr_code/pretty_qr_code.dart';
 import 'package:pyramid/core/app_state.dart';
 import 'package:pyramid/core/matrix_client.dart';
 import 'package:pyramid/core/theme.dart';
-import 'package:pyramid/core/voip_manager.dart';
 import 'package:pyramid/features/auth/bootstrap_dialog.dart';
 import 'package:pyramid/features/call/voice_channel.dart';
+import 'package:pyramid/features/call_signaling/call_signaling_service.dart';
+import 'package:pyramid/features/voice_channel/voice_channel_service.dart';
 import 'package:pyramid/features/chat/chat_view.dart';
 import 'package:pyramid/features/members/members_panel.dart';
 import 'package:pyramid/features/rooms/rooms_panel.dart';
@@ -160,7 +161,7 @@ class _AppShellState extends ConsumerState<AppShell>
     // notifyListeners (u.a. Render-Timer im Sekundentakt); ohne Flankenerkennung
     // würde der Nutzer permanent zurück in die Call-UI gezwungen und könnte
     // sie nie minimieren.
-    ref.listen(voipStateProvider, (prev, next) {
+    ref.listen(callSignalingProvider, (prev, next) {
       final wasConnected = _wasVoipConnected;
       final isConnected = next.currentCall != null &&
           next.currentCall!.state == CallState.kConnected;
@@ -172,7 +173,7 @@ class _AppShellState extends ConsumerState<AppShell>
       }
     });
 
-    ref.listen(callStateProvider, (prev, next) {
+    ref.listen(voiceChannelProvider, (prev, next) {
       // Only auto-switch to voice view for video/audio calls, NOT voice channels
       // (voice channels should stay in background with a mini bar)
       final wasActive = _wasLiveKitActive;
@@ -200,7 +201,7 @@ class _AppShellState extends ConsumerState<AppShell>
     final activeModal = ref.watch(activeModalProvider);
     final membersOpen = ref.watch(membersPanelOpenProvider);
     final innerPt = PyramidTheme.of(context);
-    final voip = ref.watch(voipStateProvider);
+    final voip = ref.watch(callSignalingProvider);
 
     final size = MediaQuery.sizeOf(context);
     final isMobile = size.width < 900 || size.height < 500;
@@ -332,7 +333,7 @@ class _MobileShell extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
-    final voip = ref.watch(voipStateProvider);
+    final voip = ref.watch(callSignalingProvider);
 
     Widget body;
     if (viewMode == ViewMode.voice) {
@@ -456,7 +457,7 @@ class _NotificationBlockedBanner extends ConsumerWidget {
 
 class _IncomingCallOverlay extends ConsumerWidget {
   final CallSession call;
-  final PyramidVoipManager voip;
+  final CallSignalingService voip;
   final PyramidTheme pt;
 
   const _IncomingCallOverlay({