quite a few UX improvements

frontend/lib/pages/onboarding.dart

@@ -4,6 +4,7 @@ import 'package:anyway/constants.dart';
 import 'package:anyway/modules/onbarding_agreement_card.dart';
 import 'package:anyway/modules/onboarding_card.dart';
 import 'package:anyway/pages/new_trip_location.dart';
+import 'package:anyway/structs/agreement.dart';
 import 'package:flutter/material.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 
@@ -121,11 +122,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
         );
       } else {
         // only allow the user to proceed if they have agreed to the terms and conditions
-        Future<bool> hasAgreed = SharedPreferences.getInstance().then(
-          (SharedPreferences prefs) {
-            return prefs.getBool('TC_agree') ?? false;
-          }
-        );
+        Future<bool> hasAgreed = getAgreement().then((agreement) => agreement.agreed);
 
         return FutureBuilder(
           future: hasAgreed,
@@ -157,8 +154,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
   );
 
   void onAgreementChanged(bool value) async {
-    SharedPreferences prefs = await SharedPreferences.getInstance();
-    await prefs.setBool('TC_agree', value);
+    saveAgreement(value);
     // Update the state of the OnboardingPage
     setState(() {});
   }