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/CatalogsGrid.razor

@@ -38,6 +38,7 @@ else
 
     <div class="grid-section">
         <DxGrid Data="@items"
+                ColumnChooserButtonDisplayMode="GridColumnChooserButtonDisplayMode.Always"
                 TItem="CatalogReadDto"
                 KeyFieldName="@nameof(CatalogReadDto.Guid)"
                 SizeMode="@_sizeMode"
@@ -61,6 +62,14 @@ else
                 @ref="gridRef">
             <ToolbarTemplate>
                 <DxToolbar>
+                    <DxToolbarItem Alignment="ToolbarItemAlignment.Right">
+                        <Template Context="_">
+                            <DxButton Text="Spalten"
+                                      RenderStyle="ButtonRenderStyle.Secondary"
+                                      RenderStyleMode="ButtonRenderStyleMode.Text"
+                                      Click="@(() => gridRef!.ShowColumnChooser())" />
+                        </Template>
+                    </DxToolbarItem>
                     <DxToolbarItem Alignment="ToolbarItemAlignment.Right">
                         <Template Context="_">
                             <DxDropDownButton Text="@FormatSizeText(_sizeMode)"