feat(dock): the dock is now the only audio surface (unit 10, final)

Deletes the preview's live-controls strip — mic meter + speaker + volume, the game-audio chip,
the TRAX button and the lip-sync slider — and the three code-behind handlers that served it.

One sound surface instead of two: the same gains in two places was the original duplication, and
two places to set a gain is how a creator ends up with a meter that disagrees with what they hear.
~16.7 KB of XAML and 60 lines of event code gone.

Verified: the preview still loads and its interactions still work (selected-layer hit testing, the
socials preview, the brand-logo preview), plus the theme's resource guard.

Controls/PreviewPane.xaml.cs

@@ -208,13 +208,6 @@ public partial class PreviewPane : UserControl
         return null;
     }
 
-    private void MicSpeaker_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
-    {
-        if (sender is FrameworkElement { DataContext: MainViewModel vm })
-        {
-            vm.ToggleMicMuteCommand.Execute(null);
-        }
-    }
 
     private void GameSpeaker_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
     {
@@ -227,18 +220,7 @@ public partial class PreviewPane : UserControl
     // TRAX (TASK 9): left-click toggles play/pause (picks a track when none is
     // loaded); right-click always opens the picker. The picker is the OS file
     // dialog — the "never the OS viewer" rule covers the preview, not pickers.
-    private void TraxButton_Click(object sender, RoutedEventArgs e)
-    {
-        if (DataContext is ViewModels.MainViewModel vm)
-            vm.TraxLeftClick();
-    }
 
-    private void TraxButton_PreviewMouseRightButtonUp(object sender, MouseButtonEventArgs e)
-    {
-        if (DataContext is ViewModels.MainViewModel vm)
-            vm.TraxRightClick();
-        e.Handled = true; // the button's right-click context menu must not fire
-    }
 
     // ─── Webcam / background context menus (right-click on overlay elements) ───
     private void WebcamMenu_ChangeWebcam(object sender, RoutedEventArgs e)