feat: auto-end call when remote party leaves

Listens for ParticipantDisconnectedEvent on the LiveKit Room. When no
remote participants remain (1:1 call ended by other side), waits 5
seconds then auto-hangs up. The delay allows for brief reconnections.

Also ensures our app properly cleans up when Element X user hangs up
on their end — our side won't sit connected to an empty LiveKit room.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/features/calls/presentation/call_controller.dart

@@ -1,9 +1,10 @@
-// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-02
+// Version: 1.3.0 | Created: 2026-04-01 | Updated: 2026-04-03
 // Call controller — manages LiveKit room connection lifecycle.
 // Transitions through idle → connecting → active → ended states.
 
 import 'dart:async';
 
+import 'package:flutter/foundation.dart';
 import 'package:livekit_client/livekit_client.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
@@ -16,12 +17,15 @@ part 'call_controller.g.dart';
 class CallController extends _$CallController {
   Timer? _durationTimer;
   Duration _elapsed = Duration.zero;
+  EventsListener<RoomEvent>? _roomListener;
 
   @override
   CallState build() {
     ref.onDispose(() {
       _durationTimer?.cancel();
       _durationTimer = null;
+      _roomListener?.dispose();
+      _roomListener = null;
       ref.read(liveKitServiceProvider).disconnect();
     });
     return const CallState.idle();
@@ -112,5 +116,26 @@ class CallController extends _$CallController {
         state = current.copyWith(duration: _elapsed);
       }
     });
+
+    // Auto-end when the remote party leaves (1:1 call behaviour).
+    // Wait 5 seconds after the last remote participant disconnects before
+    // ending — gives time for brief network interruptions to recover.
+    _roomListener?.dispose();
+    _roomListener = room.createListener();
+    _roomListener!.on<ParticipantDisconnectedEvent>((event) {
+      debugPrint(
+        '[Call] Participant left: ${event.participant.identity}, '
+        'remaining: ${room.remoteParticipants.length}',
+      );
+      if (room.remoteParticipants.isEmpty) {
+        debugPrint('[Call] No remote participants — auto-ending in 5s');
+        Future.delayed(const Duration(seconds: 5), () {
+          // Re-check in case someone rejoined during the delay.
+          if (room.remoteParticipants.isEmpty && state is CallActive) {
+            endCall();
+          }
+        });
+      }
+    });
   }
 }