DeliveryAdminWindow: Add filter for branch when in Receipt mode

Elwig/App.xaml.cs

@@ -35,6 +35,7 @@ namespace Elwig {
             }
         }
 
+        public static int BranchNum { get; private set; }
         public static string ZwstId { get; private set; }
         public static string BranchName { get; private set; }
         public static int? BranchPlz { get; private set; }
@@ -91,6 +92,7 @@ namespace Elwig {
                     Shutdown();
                     return;
                 }
+                BranchNum = branches.Count;
             }
 
             Utils.RunBackground("HTML Initialization", () => Html.Init(PrintingReadyChanged));