Fixed toJson methods

lib/data/dto/game.dart

@@ -35,7 +35,11 @@ class Game {
       winner = json['winner'] ?? '';
 
   /// Converts the Game instance to a JSON object.
-  String toJson() {
-    return 'Game{id: $id,name: $name,players: $players,group: $group,winner: $winner}';
-  }
+  Map<String, dynamic> toJson() => {
+    'id': id,
+    'name': name,
+    'players': players?.map((player) => player.toJson()).toList(),
+    'group': group?.toJson(),
+    'winner': winner,
+  };
 }

lib/data/dto/group.dart

@@ -23,7 +23,9 @@ class Group {
           .toList();
 
   /// Converts the Group instance to a JSON object.
-  String toJson() {
-    return 'Group{id: $id, name: $name,members: $members}';
-  }
+  Map<String, dynamic> toJson() => {
+    'id': id,
+    'name': name,
+    'members': members.map((member) => member.toJson()).toList(),
+  };
 }

lib/data/dto/player.dart

@@ -17,7 +17,5 @@ class Player {
       name = json['name'];
 
   /// Converts the Player instance to a JSON object.
-  String toJson() {
-    return 'Player{id: $id,name: $name}';
-  }
+  Map<String, dynamic> toJson() => {'id': id, 'name': name};
 }