Elwig: Rename buchführend, funktionär, volllieferant, and gebunden

Elwig/ViewModels/DeliveryAdminViewModel.cs

@@ -157,7 +157,7 @@ namespace Elwig.ViewModels {
         [ObservableProperty]
         public partial IEnumerable<object> WineRdSource { get; set; } = [];
         [ObservableProperty]
-        public partial bool? IsGebunden { get; set; }
+        public partial bool? IsContracted { get; set; }
 
         public ObservableCollection<Modifier> Modifiers { get; set; } = [];
         [ObservableProperty]