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.

test/services/notification_payload_parser_test.dart

@@ -0,0 +1,42 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:travel_mate/services/notification_payload_parser.dart';
+
+void main() {
+  group('NotificationPayloadParser', () {
+    test('uses explicit type when present', () {
+      final action = NotificationPayloadParser.parse({
+        'type': 'activity',
+        'tripId': 'trip_1',
+        'activityId': 'activity_42',
+      });
+
+      expect(action.type, 'activity');
+      expect(action.tripId, 'trip_1');
+      expect(action.activityId, 'activity_42');
+    });
+
+    test('infers invitation type when invitationId exists', () {
+      final action = NotificationPayloadParser.parse({
+        'invitationId': 'inv_1',
+        'tripId': 'trip_1',
+      });
+
+      expect(action.type, 'trip_invitation');
+      expect(action.invitationId, 'inv_1');
+    });
+
+    test('infers message type when groupId exists', () {
+      final action = NotificationPayloadParser.parse({'groupId': 'group_1'});
+
+      expect(action.type, 'message');
+      expect(action.groupId, 'group_1');
+    });
+
+    test('falls back to trip type when only tripId exists', () {
+      final action = NotificationPayloadParser.parse({'tripId': 'trip_1'});
+
+      expect(action.type, 'trip');
+      expect(action.tripId, 'trip_1');
+    });
+  });
+}