fix: creator feedback batch Branch 1 — TRAX volume, tooltip, meter boost, slider click, backdrop

- TRAX: LocalGain now applies 0.2× factor so music is quieter relative to desktop audio slider
- TRAX tooltip: switched to ToolTipService with Placement=Top to fix z-order in Viewbox overlay
- Mic meter: MeterLevel multiplied by 1.2× before clamping for ~20% higher visual readout
- Volume sliders: click anywhere on track to jump thumb to that position before dragging
- Backdrop: Image Visibility now binds to BackdropVisible; compositor checks IsVisible before blit
- Renamed 'Backdrop' to 'Game Capture' in EnsureBackdrop + test assertions

MainWindow.xaml.cs

@@ -83,6 +83,8 @@ public partial class MainWindow : Window
 
     private void VolumeSlider_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
     {
+        if (sender is Slider slider)
+            SetSliderValueFromClick(slider, e);
         if (sender is FrameworkElement { DataContext: MainViewModel vm })
             vm.SetVolumeAdjusting(true);
     }
@@ -101,6 +103,8 @@ public partial class MainWindow : Window
 
     private void GameVolumeSlider_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
     {
+        if (sender is Slider slider)
+            SetSliderValueFromClick(slider, e);
         if (sender is FrameworkElement { DataContext: MainViewModel vm })
             vm.SetGameVolumeAdjusting(true);
     }
@@ -117,6 +121,15 @@ public partial class MainWindow : Window
             vm.SetGameVolumeAdjusting(false);
     }
 
+    private static void SetSliderValueFromClick(Slider slider, MouseButtonEventArgs e)
+    {
+        var track = slider.Template.FindName("PART_Track", slider) as System.Windows.Controls.Primitives.Track;
+        if (track == null) return;
+        var pos = e.GetPosition(track);
+        var fraction = Math.Clamp(pos.X / track.RenderSize.Width, 0, 1);
+        slider.Value = slider.Minimum + fraction * (slider.Maximum - slider.Minimum);
+    }
+
     private void MicSpeaker_MouseLeftButtonUp(object sender, MouseButtonEventArgs e)
     {
         if (sender is FrameworkElement { DataContext: MainViewModel vm })