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

frontend/lib/domain/entities/landmark.dart

@@ -48,3 +48,9 @@ abstract class Landmark with _$Landmark {
 
   factory Landmark.fromJson(Map<String, Object?> json) => _$LandmarkFromJson(json);
 }
+
+extension LandmarkVisitX on Landmark {
+  bool get isVisited => visited ?? false;
+
+  set isVisited(bool value) => visited = value;
+}