feat: Implement trip invitation functionality and notification handling

- Added TripInvitationRepository for managing trip invitations.
- Created TripInvitation model with serialization methods.
- Implemented notification payload parser for handling FCM notifications.
- Enhanced NotificationService to manage trip invitations and related actions.
- Updated UserRepository to include user search functionality.
- Modified AuthRepository to store multiple FCM tokens.
- Added tests for trip invitation logic and notification payload parsing.
- Updated pubspec.yaml and pubspec.lock for dependency management.

lib/components/activities/activities_page.dart

@@ -926,7 +926,7 @@ class _ActivitiesPageState extends State<ActivitiesPage>
                           const SizedBox(width: 8),
                           Expanded(
                             child: Text(
-                              'Activité éloignée : ${distanceInKm!.toStringAsFixed(0)} km du lieu du voyage',
+                              'Activité éloignée : ${distanceInKm.toStringAsFixed(0)} km du lieu du voyage',
                               style: theme.textTheme.bodySmall?.copyWith(
                                 color: theme.colorScheme.onErrorContainer,
                                 fontWeight: FontWeight.w600,