Add system mute control via setMute action

Introduce a setMute protocol action and an NAudio-backed
AudioControlService so the Android client can toggle the Windows
render endpoint mute state over WebSocket.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

WirelessTextSyncer.Windows/Services/AudioControlService.cs

@@ -0,0 +1,52 @@
+using System.Runtime.InteropServices;
+using NAudio.CoreAudioApi;
+
+namespace WirelessTextSyncer.Windows.Services;
+
+public sealed class AudioControlService : IAudioControlService, IDisposable
+{
+    private readonly MMDeviceEnumerator enumerator = new();
+    private bool disposed;
+
+    public bool IsMuted
+    {
+        get
+        {
+            try
+            {
+                using var device = enumerator.GetDefaultAudioEndpoint(DataFlow.Render, Role.Multimedia);
+                return device.AudioEndpointVolume.Mute;
+            }
+            catch (COMException exception)
+            {
+                AppLogger.Error("Failed to read audio mute state.", exception);
+                return false;
+            }
+        }
+    }
+
+    public void SetMute(bool muted)
+    {
+        try
+        {
+            using var device = enumerator.GetDefaultAudioEndpoint(DataFlow.Render, Role.Multimedia);
+            device.AudioEndpointVolume.Mute = muted;
+            AppLogger.Info($"Audio mute set to {muted}.");
+        }
+        catch (COMException exception)
+        {
+            AppLogger.Error($"Failed to set audio mute to {muted}.", exception);
+        }
+    }
+
+    public void Dispose()
+    {
+        if (disposed)
+        {
+            return;
+        }
+
+        disposed = true;
+        enumerator.Dispose();
+    }
+}

WirelessTextSyncer.Windows/Services/IAudioControlService.cs

@@ -0,0 +1,8 @@
+namespace WirelessTextSyncer.Windows.Services;
+
+public interface IAudioControlService
+{
+    bool IsMuted { get; }
+
+    void SetMute(bool muted);
+}

WirelessTextSyncer.Windows/Services/SyncMessageHandler.cs

@@ -5,11 +5,13 @@ namespace WirelessTextSyncer.Windows.Services;
 public sealed class SyncMessageHandler
 {
     private readonly IKeyboardInjectionService keyboard;
+    private readonly IAudioControlService audio;
     private string remoteText = string.Empty;
 
-    public SyncMessageHandler(IKeyboardInjectionService keyboard)
+    public SyncMessageHandler(IKeyboardInjectionService keyboard, IAudioControlService audio)
     {
         this.keyboard = keyboard;
+        this.audio = audio;
     }
 
     public void Handle(SyncMessage message)
@@ -38,11 +40,21 @@ public sealed class SyncMessageHandler
                 break;
             case SyncAction.Ping:
                 break;
+            case SyncAction.SetMute:
+                HandleSetMute(message);
+                break;
             default:
                 throw new InvalidOperationException($"Unsupported sync action: {message.Action}");
         }
     }
 
+    private void HandleSetMute(SyncMessage message)
+    {
+        var muted = message.Muted ?? false;
+        AppLogger.Info($"SetMute received muted={muted}.");
+        audio.SetMute(muted);
+    }
+
     private void AppendSubmittedText(string text)
     {
         if (string.IsNullOrEmpty(text))