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

ViewModels/MainViewModel.cs

@@ -197,10 +197,15 @@ public class MainViewModel : ViewModelBase
         private set
         {
             if (SetProperty(ref _backdropImage, value))
+            {
                 OnPropertyChanged(nameof(ShowPreviewPlaceholder));
+                OnPropertyChanged(nameof(BackdropVisible));
+            }
         }
     }
 
+    public bool BackdropVisible => ActiveScene?.Elements.OfType<Source>().FirstOrDefault(s => s.IsBackdrop)?.IsVisible == true;
+
     public SceneElement? SelectedElement
     {
         get => _selectedElement;
@@ -337,7 +342,7 @@ public class MainViewModel : ViewModelBase
     /// level flashes on the bar) so the creator sees where they're setting it;
     /// otherwise the realtime live level scaled by volume (raising the volume
     /// moves ambient noise up the bar). Read-only meter.</summary>
-    private double MeterLevel => MicMuted ? 0 : _volumeAdjusting || _volumeFlash ? MicVolume : Math.Min(1, AudioLevelMeter.ToDisplay((float)AudioLevel) * MicVolume);
+    private double MeterLevel => MicMuted ? 0 : _volumeAdjusting || _volumeFlash ? MicVolume : Math.Min(1, AudioLevelMeter.ToDisplay((float)AudioLevel) * 1.2 * MicVolume);
 
     public double MeterFillWidth => MeterLevel * 288;
 
@@ -595,7 +600,7 @@ public class MainViewModel : ViewModelBase
     /// Called whenever the game volume/mute change and on TRAX load.</summary>
     private void UpdateTraxLocalGain()
     {
-        _musicPlayer.LocalGain = GameMuted ? 0f : (float)GameAudioVolume;
+        _musicPlayer.LocalGain = GameMuted ? 0f : (float)(GameAudioVolume * 0.2);
     }
 
     /// <summary>Live desktop/game input level (0..1), fed by the mixer's loopback
@@ -1405,7 +1410,7 @@ public class MainViewModel : ViewModelBase
 
         backdrop = new Source
         {
-            Name = "Backdrop",
+            Name = "Game Capture",
             Type = SourceType.DisplayCapture,
             IsBackdrop = true,
             IsEnabled = true,
@@ -1697,6 +1702,8 @@ public class MainViewModel : ViewModelBase
     {
         if (element == null) return;
         element.IsVisible = !element.IsVisible;
+        if (element is Source { IsBackdrop: true })
+            OnPropertyChanged(nameof(BackdropVisible));
     }
 
     private void AddSource(object? parameter)