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/pages/trip_invitations_page_test.dart

@@ -0,0 +1,124 @@
+import 'dart:async';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:travel_mate/models/trip_invitation.dart';
+import 'package:travel_mate/pages/trip_invitations_page.dart';
+import 'package:travel_mate/repositories/trip_invitation_repository.dart';
+
+class _FakeTripInvitationDataSource implements TripInvitationDataSource {
+  final StreamController<List<TripInvitation>> _controller =
+      StreamController<List<TripInvitation>>();
+
+  bool acceptedCalled = false;
+  bool rejectedCalled = false;
+
+  void emitInvitations(List<TripInvitation> invitations) {
+    _controller.add(invitations);
+  }
+
+  @override
+  Future<void> respondToInvitation({
+    required String invitationId,
+    required bool isAccepted,
+  }) async {
+    if (isAccepted) {
+      acceptedCalled = true;
+      return;
+    }
+    rejectedCalled = true;
+  }
+
+  @override
+  Stream<List<TripInvitation>> watchInvitationsForUser(String userId) {
+    return _controller.stream;
+  }
+
+  @override
+  Future<void> createInvitation({
+    required String tripId,
+    required String tripTitle,
+    required String inviterId,
+    required String inviterName,
+    required String inviteeId,
+    required String inviteeEmail,
+  }) async {}
+
+  @override
+  Future<TripInvitation?> getInvitationById(String invitationId) async {
+    return null;
+  }
+
+  @override
+  Future<TripInvitation?> getPendingInvitation({
+    required String tripId,
+    required String inviteeId,
+  }) async {
+    return null;
+  }
+
+  @override
+  Stream<List<TripInvitation>> watchPendingInvitationsForUser(String userId) {
+    return _controller.stream;
+  }
+
+  Future<void> dispose() async {
+    await _controller.close();
+  }
+}
+
+void main() {
+  group('TripInvitationsPage', () {
+    testWidgets('shows invitations in correct tabs', (tester) async {
+      final fakeRepository = _FakeTripInvitationDataSource();
+
+      await tester.pumpWidget(
+        MaterialApp(
+          home: TripInvitationsPage(
+            repository: fakeRepository,
+            userIdOverride: 'user_1',
+          ),
+        ),
+      );
+
+      final invitations = [
+        TripInvitation(
+          id: 'pending_1',
+          tripId: 'trip_1',
+          tripTitle: 'Road Trip',
+          inviterId: 'u2',
+          inviterName: 'Alice',
+          inviteeId: 'user_1',
+          inviteeEmail: 'user@example.com',
+          status: 'pending',
+          createdAt: DateTime(2026, 3, 6),
+        ),
+        TripInvitation(
+          id: 'accepted_1',
+          tripId: 'trip_2',
+          tripTitle: 'Tokyo',
+          inviterId: 'u3',
+          inviterName: 'Bob',
+          inviteeId: 'user_1',
+          inviteeEmail: 'user@example.com',
+          status: 'accepted',
+          createdAt: DateTime(2026, 3, 6),
+        ),
+      ];
+
+      fakeRepository.emitInvitations(invitations);
+      await tester.pump();
+
+      expect(find.text('Road Trip'), findsOneWidget);
+      expect(find.text('Accepter'), findsOneWidget);
+
+      await tester.tap(find.text('Acceptées'));
+      await tester.pumpAndSettle();
+
+      expect(find.text('Tokyo'), findsOneWidget);
+      expect(find.text('Accepter'), findsNothing);
+
+      await fakeRepository.dispose();
+    });
+  });
+}