Add centre-panned vocal reduction to live and saved audio

frontend/index.html

@@ -660,6 +660,7 @@
     <script src="lyrics-window.js"></script>
     <script src="midi.js"></script>
     <script src="lower-third.js"></script>
+    <script src="vocal-reducer.js"></script>
     <script src="app.js"></script>
   </body>
 </html>