overhaul using a trip struct that notifies its ui dependencies

frontend/lib/utils/fetch_trip.dart

@@ -1,7 +1,7 @@
 import "dart:convert";
 import "dart:developer";
-
 import 'package:dio/dio.dart';
+
 import 'package:anyway/constants.dart';
 import "package:anyway/structs/landmark.dart";
 import "package:anyway/structs/trip.dart";
@@ -25,14 +25,14 @@ Dio dio = Dio(
   ),
 );
 
-Future<Trip>? fetchTrip(
-  List<double> startPoint,
+fetchTrip(
+  Trip trip,
   Future<UserPreferences> preferences,
 ) async {
   UserPreferences prefs = await preferences;
   Map<String, dynamic> data = {
     "preferences": prefs.toJson(),
-    "start": startPoint
+    "start": trip.landmarks!.first.location,
   };
   String dataString = jsonEncode(data);
   log(dataString);
@@ -44,24 +44,25 @@ Future<Trip>? fetchTrip(
 
   // handle errors
   if (response.statusCode != 200) {
+    trip.updateUUID("error");
     throw Exception('Failed to load trip');
   }
   if (response.data["error"] != null) {
+    trip.updateUUID("error");
     throw Exception(response.data["error"]);
   }
   log(response.data.toString());
   Map<String, dynamic> json = response.data;
 
-  // only fetch the trip "meta" data for now
-  Trip trip = Trip.fromJson(json);
+  // only fill in the trip "meta" data for now
+  trip.loadFromJson(json);
 
   String? nextUUID = json["first_landmark_uuid"];
   while (nextUUID != null) {
     var (landmark, newUUID) = await fetchLandmark(nextUUID);
-    trip.landmarks.add(landmark);
+    trip.addLandmark(landmark);
     nextUUID = newUUID;
   }
-  return trip;
 }
 
 

frontend/lib/utils/load_trips.dart

@@ -17,7 +17,7 @@ Future<List<Trip>> loadTrips() async {
   }
 
   if (trips.isEmpty) {
-    Trip t1 = Trip(uuid: '1', cityName: 'Paris', landmarks: LinkedList<Landmark>());
+    Trip t1 = Trip(uuid: '1', landmarks: LinkedList<Landmark>());
     t1.landmarks.add(
       Landmark(
         uuid: '1',
@@ -66,7 +66,7 @@ Future<List<Trip>> loadTrips() async {
     trips.add(t1);
 
 
-    Trip t2 = Trip(uuid: '2', cityName: 'Vienna', landmarks: LinkedList<Landmark>());
+    Trip t2 = Trip(uuid: '2', landmarks: LinkedList<Landmark>());
 
     t2.landmarks.add(
       Landmark(