Documents/Pdf: Add progress tracker

Elwig/Windows/MemberAdminWindow.xaml.cs

@@ -317,7 +317,7 @@ namespace Elwig.Windows {
                         .ThenBy(m => m.MgNr);
                     break;
             }
-            using var doc = await Document.Merge((await members.ToListAsync()).Select(m => new Letterhead(m)));
+            using var doc = Document.Merge((await members.ToListAsync()).Select(m => new Letterhead(m)));
             await doc.Generate();
             Mouse.OverrideCursor = null;
             if (App.Config.Debug) {