ContextWindow: Add HasContextLoaded property

Elwig/Windows/MainWindow.xaml.cs

@@ -41,6 +41,7 @@ namespace Elwig.Windows {
         }
 
         protected override async Task OnInit(AppDbContext ctx) {
+            await base.OnInit(ctx);
             if (Utils.HasInternetConnectivity()) {
                 CheckSync(200);
             }