fix: resolve post-milestone tech debt items

- Add DataGrid RowStyle with red highlighting for invalid CSV rows
  in BulkMembersView, BulkSitesView, and FolderStructureView
- Fix cancel test locale mismatch by setting EN culture before assertion
- Remove dead FeatureTabBase placeholder (replaced by full tab views)
- Clean up unused xmlns:controls from MainWindow.xaml

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

SharepointToolbox/Views/Tabs/BulkSitesView.xaml

@@ -31,6 +31,16 @@
 
         <DataGrid ItemsSource="{Binding PreviewRows}" AutoGenerateColumns="False"
                   IsReadOnly="True" CanUserSortColumns="True">
+            <DataGrid.RowStyle>
+                <Style TargetType="DataGridRow">
+                    <Style.Triggers>
+                        <DataTrigger Binding="{Binding IsValid}" Value="False">
+                            <Setter Property="Background" Value="#FFFDE0E0" />
+                            <Setter Property="Foreground" Value="#FFB00020" />
+                        </DataTrigger>
+                    </Style.Triggers>
+                </Style>
+            </DataGrid.RowStyle>
             <DataGrid.Columns>
                 <DataGridTextColumn Header="Valid" Binding="{Binding IsValid}" Width="50" />
                 <DataGridTextColumn Header="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[bulksites.name]}"