Enhance grid UI: column chooser button & default widths

- Set default width and hide new button for DxGridBandColumn in BandGridBase.cs
- Always show column chooser button in CatalogsGrid and MassDataGrid
- Add "Spalten" toolbar button to open column chooser dialog
- Improves accessibility and consistency of grid column customization

DbFirst.BlazorWebApp/Components/BandGridBase.cs

@@ -184,6 +184,8 @@ public abstract class BandGridBase<TItem> : ComponentBase
         {
             if (band.Columns.Count == 0) continue;
             builder.OpenComponent<DxGridBandColumn>(seq++);
+            builder.AddAttribute(seq++, "Width", "120px");
+            builder.AddAttribute(seq++, "NewButtonVisible", false); // falls noch nicht vorhanden
             builder.AddAttribute(seq++, "Caption", band.Caption);
             builder.AddAttribute(seq++, "Columns", (RenderFragment)(bandBuilder =>
             {