Refactor LoginPage to MVVM

Jugenddienst Stunden/Converter/InverseBoolConverter.cs

@@ -0,0 +1,15 @@
+using System.Globalization;
+
+namespace Jugenddienst_Stunden.Converter;
+
+public sealed class InverseBoolConverter : IValueConverter {
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
+        if (value is bool b) return !b;
+        return true;
+    }
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
+        if (value is bool b) return !b;
+        return false;
+    }
+}