Clean up two warnings from PR #336 (#339)

- Replace deprecated DataGridRow.GetIndex() with the Index property
- Remove dead _suppressGridSelectionEvent field; the suppression was
  only needed for the old ItemsSource-reset highlight path, which was
  refactored away

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erikdarlingdata

src/PlanViewer.App/Controls/QueryStoreHistoryControl.axaml.cs

@@ -47,7 +47,6 @@ public partial class QueryStoreHistoryControl : UserControl
 
 	// Legend state
 	private bool _legendExpanded;
-	private bool _suppressGridSelectionEvent;
 	private bool _isLoadingPlan;
 	private string _dataSummaryText = "";
 
@@ -793,7 +792,7 @@ private void HighlightGridRows()
 		// (which would wipe sort state and scroll position)
 		foreach (var row in HistoryDataGrid.GetVisualDescendants().OfType<DataGridRow>())
 		{
-			var idx = row.GetIndex();
+			var idx = row.Index;
 			row.Background = _selectedRowIndices.Contains(idx)
 				? new SolidColorBrush(Avalonia.Media.Color.FromArgb(60, 79, 195, 247))
 				: Brushes.Transparent;
@@ -829,8 +828,6 @@ private void OnHighlightLoadingRow(object? sender, DataGridRowEventArgs e)
 
 	private void HistoryDataGrid_SelectionChanged(object? sender, SelectionChangedEventArgs e)
 	{
-		if (_suppressGridSelectionEvent) return;
-
 		_selectedRowIndices.Clear();
 		if (HistoryDataGrid.SelectedItems != null)
 		{