creator feedback batch: 9 fixes from initial review

1. About box bg → #0A0C1A (matches icon bg)
2. Elements panel collapses when no element selected / scene changes
3. OpacityChip moved from preview overlay into Elements panel
4. Color picker fix: cast to SceneElement not Source (WebcamSceneConfig was silently blocked)
5. Red left accent bar on selected items in Scene/Source lists
6. ✓ character replaced with Path checkmark icon in Elements panel
7. Removed ToolTip='Applied' from green check buttons
8. Created bugs.md with first entry (iTunes audio on mic meter)
9. Full logo in README + About box uses ytLlive-logo.png

Helpers/NullToVisibilityConverter.cs

@@ -0,0 +1,14 @@
+using System.Globalization;
+using System.Windows;
+using System.Windows.Data;
+
+namespace ytLive.Helpers;
+
+public class NullToVisibilityConverter : IValueConverter
+{
+    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        => value == null ? Visibility.Visible : Visibility.Collapsed;
+
+    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        => value is Visibility.Collapsed;
+}