account for changed itineraries once landmarks are marked as done or deleted

frontend/lib/modules/step_between_landmarks.dart

@@ -22,6 +22,14 @@ class _StepBetweenLandmarksState extends State<StepBetweenLandmarks> {
   @override
   Widget build(BuildContext context) {
     int? time = widget.current.tripTime?.inMinutes;
+    
+    // since landmarks might have been marked as visited, the next landmark might not be the immediate next in the list
+    // => the precomputed trip time is not valid anymore
+    if (widget.current.next != widget.next) {
+      time = null;
+    }
+    
+    // round 0 travel time to 1 minute
     if (time != null && time < 1) {
       time = 1;
     }