Fix: desktop audio slider now controls system volume (WYSIWYG audio)

Push GameAudioVolume to MMDevice.AudioEndpointVolume.MasterVolumeLevelScalar
on every change. The slider now controls what Windows sends to the
headphones, so the creator hears exactly what the stream carries (minus
mic — monitored via meters only, no feedback).

207 tests, 0 warnings.

ViewModels/MainViewModel.cs

@@ -11,6 +11,7 @@ using System.Windows.Media;
 using System.Windows.Media.Imaging;
 using System.Windows.Threading;
 using Microsoft.Win32;
+using NAudio.CoreAudioApi;
 using ytLive.Helpers;
 using ytLive.Models;
 using ytLive.Services;
@@ -612,6 +613,17 @@ public class MainViewModel : ViewModelBase
         _musicPlayer.LocalGain = GameMuted ? 0f : (float)(GameAudioVolume * 0.2);
     }
 
+    private static void PushSystemVolume(double volume)
+    {
+        try
+        {
+            using var enumerator = new MMDeviceEnumerator();
+            using var device = enumerator.GetDefaultAudioEndpoint(DataFlow.Render, Role.Multimedia);
+            device.AudioEndpointVolume.MasterVolumeLevelScalar = (float)volume;
+        }
+        catch { }
+    }
+
     /// <summary>Live desktop/game input level (0..1), fed by the mixer's loopback
     /// capture. Read by the game meter, scaled by GameAudioVolume.</summary>
     public double GameAudioLevel
@@ -702,6 +714,7 @@ public class MainViewModel : ViewModelBase
                 OnPropertyChanged(nameof(GameMeterFillWidth));
                 OnPropertyChanged(nameof(GameMeterBrush));
                 UpdateTraxLocalGain();
+                PushSystemVolume(clamped);
             }
         }
     }