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.Tests/ViewModels/FeatureViewModelBaseTests.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using Microsoft.Extensions.Logging.Abstractions;
 using SharepointToolbox.Core.Models;
 using SharepointToolbox.ViewModels;
@@ -61,23 +62,33 @@ public class FeatureViewModelBaseTests
     [Fact]
     public async Task CancelCommand_DuringOperation_SetsStatusMessageToCancelled()
     {
-        var vm = new TestViewModel();
-        var started = new TaskCompletionSource<bool>();
-
-        vm.OperationFunc = async (ct, p) =>
+        // Ensure EN culture so TranslationSource resolves "Operation cancelled"
+        var prev = CultureInfo.CurrentUICulture;
+        CultureInfo.CurrentUICulture = CultureInfo.GetCultureInfo("en");
+        try
         {
-            started.SetResult(true);
-            await Task.Delay(5000, ct); // Will be cancelled
-        };
+            var vm = new TestViewModel();
+            var started = new TaskCompletionSource<bool>();
 
-        var runTask = vm.RunCommand.ExecuteAsync(null);
-        await started.Task;
+            vm.OperationFunc = async (ct, p) =>
+            {
+                started.SetResult(true);
+                await Task.Delay(5000, ct); // Will be cancelled
+            };
 
-        vm.CancelCommand.Execute(null);
-        await runTask;
+            var runTask = vm.RunCommand.ExecuteAsync(null);
+            await started.Task;
 
-        Assert.Contains("cancel", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
-        Assert.False(vm.IsRunning);
+            vm.CancelCommand.Execute(null);
+            await runTask;
+
+            Assert.Contains("cancel", vm.StatusMessage, StringComparison.OrdinalIgnoreCase);
+            Assert.False(vm.IsRunning);
+        }
+        finally
+        {
+            CultureInfo.CurrentUICulture = prev;
+        }
     }
 
     [Fact]