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/MainWindow.xaml

@@ -4,7 +4,7 @@
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:loc="clr-namespace:SharepointToolbox.Localization"
-        xmlns:controls="clr-namespace:SharepointToolbox.Views.Controls"
+
         xmlns:views="clr-namespace:SharepointToolbox.Views.Tabs"
         mc:Ignorable="d"
         Title="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[app.title]}"
@@ -39,7 +39,7 @@
                  Background="Black" Foreground="LimeGreen"
                  FontFamily="Consolas" FontSize="11" />
 
-    <!-- TabControl: 7 stub tabs use FeatureTabBase; Settings tab wired in plan 01-07 -->
+    <!-- TabControl -->
     <TabControl>
       <TabItem x:Name="PermissionsTabItem"
                Header="{Binding Source={x:Static loc:TranslationSource.Instance}, Path=[tab.permissions]}">