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/SyncAction.cs

@@ -9,5 +9,6 @@ public enum SyncAction
     ReplaceAll,
     Backspace,
     Enter,
-    Ping
+    Ping,
+    SetMute
 }