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/IAudioControlService.cs

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