Fix NullReferenceException in PriceSliderCard_ValueChanged

Slider ValueChanged fires during XAML init before named controls exist.
Guard with IsLoaded check, same pattern as UpdateCardTitleBar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

EbayListingTool/Views/PhotoAnalysisView.xaml.cs

@@ -749,6 +749,7 @@ public partial class PhotoAnalysisView : UserControl
 
     private void PriceSliderCard_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
     {
+        if (!IsLoaded) return;
         var price = (decimal)e.NewValue;
         SliderVerbalLabel.Text = NumberWords.ToVerbalPrice(price);
         SliderDigitLabel.Text  = $"£{price:F2}";