Enhance group member management: add last name support in GroupMember model, update member display in chat and trip details, and implement pseudo change functionality in chat group.

lib/components/home/create_trip_content.dart

@@ -956,6 +956,7 @@ class _CreateTripContentState extends State<CreateTripContent> {
       GroupMember(
         userId: currentUser.id,
         firstName: currentUser.prenom,
+        lastName: currentUser.nom,
         pseudo: currentUser.prenom,
         role: 'admin',
         profilePictureUrl: currentUser.profilePictureUrl,
@@ -963,6 +964,7 @@ class _CreateTripContentState extends State<CreateTripContent> {
       ...participantsData.map((p) => GroupMember(
             userId: p['id'] as String,
             firstName: p['firstName'] as String,
+            lastName: p['lastName'] as String? ?? '',
             pseudo: p['firstName'] as String,
             role: 'member',
             profilePictureUrl: p['profilePictureUrl'] as String?,
@@ -1151,11 +1153,13 @@ class _CreateTripContentState extends State<CreateTripContent> {
         if (userId != null) {
           final userDoc = await _userService.getUserById(userId);
           final firstName = userDoc?.prenom ?? 'Utilisateur';
+          final lastName = userDoc?.nom ?? '';
           final profilePictureUrl = userDoc?.profilePictureUrl;
 
           participantsData.add({
             'id': userId,
             'firstName': firstName,
+            'lastName': lastName,
             'profilePictureUrl': profilePictureUrl,
           });
         } else {