Make day page switchable

lib/main.dart

@@ -66,7 +66,7 @@ class _MyHomePageState extends State<MyHomePage> {
   @override
   Widget build(BuildContext context) {
     // Manage days and create items list of a pages.
-    final days = [AdventOfCodeDays.day1, AdventOfCodeDays.day2];
+    final days = AdventOfCodeDays.values;
     List<NavigationRailDestination> dayItems = [];
     for (var day in days) {
       dayItems.add(
@@ -78,7 +78,7 @@ class _MyHomePageState extends State<MyHomePage> {
     }
     var colorScheme = Theme.of(context).colorScheme;
 
-    Widget page = DayPage(AdventOfCodeDays.day1);
+    Widget page = DayPage(days[selectedIndex]);
 
     // The container for the current page, with its background color
     // and subtle switching animation.