feat: Prevent sending messages if other party has no encryption keys

lib/utils/other_party_can_receive.dart

@@ -0,0 +1,21 @@
+import 'package:matrix/matrix.dart';
+
+extension OtherPartyCanReceiveExtension on Room {
+  bool get otherPartyCanReceiveMessages {
+    if (!encrypted) return true;
+    final users = getParticipants()
+        .map((u) => u.id)
+        .where((userId) => userId != client.userID)
+        .toSet();
+    if (users.isEmpty) return true;
+
+    for (final userId in users) {
+      if (client.userDeviceKeys[userId]?.deviceKeys.values.isNotEmpty == true) {
+        return true;
+      }
+    }
+    return false;
+  }
+}
+
+class OtherPartyCanNotReceiveMessages implements Exception {}