tentatively shrink trip overview, nicer onboarding

frontend/lib/utils/load_trips.dart

@@ -1,16 +1,39 @@
 import 'package:anyway/structs/trip.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
-Future<List<Trip>> loadTrips() async {
-  SharedPreferences prefs = await SharedPreferences.getInstance();
+import 'package:flutter/foundation.dart';
 
-  List<Trip> trips = [];
-  Set<String> keys = prefs.getKeys();
-  for (String key in keys) {
-    if (key.startsWith('trip_')) {
-      String uuid = key.replaceFirst('trip_', '');
-      trips.add(Trip.fromPrefs(prefs, uuid));
+class SavedTrips extends ChangeNotifier {
+  List<Trip> _trips = [];
+
+  List<Trip> get trips => _trips;
+
+  void loadTrips() async {
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+
+    List<Trip> trips = [];
+    Set<String> keys = prefs.getKeys();
+    for (String key in keys) {
+      if (key.startsWith('trip_')) {
+        String uuid = key.replaceFirst('trip_', '');
+        trips.add(Trip.fromPrefs(prefs, uuid));
+      }
     }
+    _trips = trips;
+    notifyListeners();
+  }
+
+  void addTrip(Trip trip) async {
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+    trip.toPrefs(prefs);
+    _trips.add(trip);
+    notifyListeners();
+  }
+
+  void clearTrips () async {
+    SharedPreferences prefs = await SharedPreferences.getInstance();
+    prefs.clear();
+    _trips = [];
+    notifyListeners();
   }
-  return trips;
 }