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/Models/SyncMessage.cs

@@ -9,4 +9,7 @@ public sealed record SyncMessage
 
     [JsonPropertyName("text")]
     public string? Text { get; init; }
+
+    [JsonPropertyName("muted")]
+    public bool? Muted { get; init; }
 }