feat: Add group creation functionality and refactor trip service for improved data handling

lib/models/group.dart

@@ -1,9 +1,7 @@
-import 'package:travel_mate/models/user.dart';
-
 class Group {
   final String? id;
   final String name;
-  final List<User> members;
+  final List<String> members;
 
   Group({
     this.id,
@@ -15,14 +13,14 @@ class Group {
     return Group(
       id: documentId,
       name: data['name'] ?? '',
-      members: List<User>.from(data['members']?.map((member) => User.fromMap(member)) ?? []),
+      members: List<String>.from(data['members'] ?? []),
     );
   }
 
   Map<String, dynamic> toMap() {
     return {
       'name': name,
-      'members': members.map((member) => member.toMap()).toList(),
+      'members': members,
     };
   }
 }