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.Tests/SyncMessageTests.cs

@@ -46,7 +46,7 @@ public sealed class SyncMessageTests
     public void ReplaceAllAppendsSubmittedText()
     {
         var keyboard = new RecordingKeyboardInjectionService();
-        var handler = new SyncMessageHandler(keyboard);
+        var handler = new SyncMessageHandler(keyboard, new RecordingAudioControlService());
 
         handler.Handle(new SyncMessage
         {
@@ -59,6 +59,33 @@ public sealed class SyncMessageTests
             keyboard.Calls);
     }
 
+    [TestMethod]
+    public void DeserializeAndroidSetMuteMessage()
+    {
+        var message = JsonSerializer.Deserialize<SyncMessage>(
+            """{"action":"setMute","muted":true}""",
+            new JsonSerializerOptions(JsonSerializerDefaults.Web));
+
+        Assert.IsNotNull(message);
+        Assert.AreEqual(SyncAction.SetMute, message.Action);
+        Assert.AreEqual(true, message.Muted);
+    }
+
+    [TestMethod]
+    public void SetMuteForwardsToAudioService()
+    {
+        var audio = new RecordingAudioControlService();
+        var handler = new SyncMessageHandler(new RecordingKeyboardInjectionService(), audio);
+
+        handler.Handle(new SyncMessage
+        {
+            Action = SyncAction.SetMute,
+            Muted = true
+        });
+
+        CollectionAssert.AreEqual(new[] { "mute:True" }, audio.Calls);
+    }
+
     [TestMethod]
     public void DiscoveryResponseContainsServiceEndpoint()
     {
@@ -77,7 +104,7 @@ public sealed class SyncMessageTests
     [TestMethod]
     public void WebSocketServiceInfoContainsDeviceName()
     {
-        using var server = new WebSocketServerService(new SyncMessageHandler(new RecordingKeyboardInjectionService()), "Desktop-WIN11");
+        using var server = new WebSocketServerService(new SyncMessageHandler(new RecordingKeyboardInjectionService(), new RecordingAudioControlService()), "Desktop-WIN11");
 
         using var document = JsonDocument.Parse(server.BuildServiceInfoJson());
         var root = document.RootElement;
@@ -112,4 +139,16 @@ public sealed class SyncMessageTests
             Calls.Add("enter");
         }
     }
+
+    private sealed class RecordingAudioControlService : IAudioControlService
+    {
+        public List<string> Calls { get; } = [];
+
+        public bool IsMuted => Calls.Count > 0 && Calls[^1] == "mute:True";
+
+        public void SetMute(bool muted)
+        {
+            Calls.Add($"mute:{muted}");
+        }
+    }
 }