feat(wip): implement trip persistence through a local repository. Include loaded trips in the start page UI

frontend/test/trip_local_datasource_test.dart

@@ -0,0 +1,69 @@
+import 'package:anyway/data/datasources/trip_local_datasource.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  late TripLocalDataSource dataSource;
+
+  Map<String, dynamic> buildTripJson(String uuid) {
+    return {
+      'uuid': uuid,
+      'total_time_minute': 90,
+      'landmarks': [
+        {
+          'uuid': 'lm-$uuid',
+          'name': 'Landmark $uuid',
+          'location': [0.0, 0.0],
+          'type': {'type': 'sightseeing'},
+        },
+      ],
+    };
+  }
+
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    dataSource = TripLocalDataSourceImpl();
+  });
+
+  test('loadTrips returns empty list when storage is empty', () async {
+    final trips = await dataSource.loadTrips();
+    expect(trips, isEmpty);
+  });
+
+  test('upsertTrip stores and retrieves a trip', () async {
+    await dataSource.upsertTrip(buildTripJson('trip-1'));
+    final trips = await dataSource.loadTrips();
+    expect(trips, hasLength(1));
+    expect(trips.first['uuid'], 'trip-1');
+
+    final fetched = await dataSource.getTrip('trip-1');
+    expect(fetched, isNotNull);
+    expect(fetched!['uuid'], 'trip-1');
+  });
+
+  test('deleteTrip removes the persisted entry', () async {
+    await dataSource.upsertTrip(buildTripJson('trip-1'));
+    await dataSource.upsertTrip(buildTripJson('trip-2'));
+
+    await dataSource.deleteTrip('trip-1');
+    final remaining = await dataSource.loadTrips();
+    expect(remaining, hasLength(1));
+    expect(remaining.single['uuid'], 'trip-2');
+  });
+
+  test('upsertTrip overwrites an existing trip and keeps latest first', () async {
+    await dataSource.upsertTrip(buildTripJson('trip-1'));
+    await dataSource.upsertTrip(buildTripJson('trip-2'));
+
+    final updatedTrip1 = buildTripJson('trip-1')..['total_time_minute'] = 120;
+    await dataSource.upsertTrip(updatedTrip1);
+
+    final trips = await dataSource.loadTrips();
+    expect(trips, hasLength(2));
+    expect(trips.first['uuid'], 'trip-1');
+    expect(trips.first['total_time_minute'], 120);
+    expect(trips[1]['uuid'], 'trip-2');
+  });
+}