more pleasant progress handling, although somewhat flawed

frontend/lib/structs/landmark.dart

@@ -27,7 +27,7 @@ final class Landmark extends LinkedListEntry<Landmark>{
   String? imageURL; // not final because it can be patched
   final String? description;
   final Duration? duration;
-  final bool? visited;
+  bool visited;
 
   // Next node
   // final Landmark? next;
@@ -46,7 +46,7 @@ final class Landmark extends LinkedListEntry<Landmark>{
     this.imageURL,
     this.description,
     this.duration,
-    this.visited,
+    this.visited = false,
 
     // this.next,
     this.tripTime,
@@ -71,7 +71,7 @@ final class Landmark extends LinkedListEntry<Landmark>{
       final imageURL = json['image_url'] as String?;
       final description = json['description'] as String?;
       var duration = Duration(minutes: json['duration'] ?? 0) as Duration?;
-      final visited = json['visited'] as bool?;
+      final visited = json['visited'] ?? false as bool;
       var tripTime = Duration(minutes: json['time_to_reach_next'] ?? 0) as Duration?;
       
       return Landmark(