voice-poc: wire up screen share button (S shortcut, active/disabled states)

voice-poc/public/index.html

@@ -37,6 +37,7 @@
       <div class="user-buttons">
         <button id="btn-mic" class="ctl" title="Toggle mic (M)">&#127908;</button>
         <button id="btn-deafen" class="ctl" title="Toggle deafen (D)">&#128064;</button>
+        <button id="btn-share" class="ctl" title="Share your screen" disabled>&#128421;&#65039;</button>
         <button id="btn-settings" class="ctl" title="Voice &amp; ICE settings">&#9881;&#65039;</button>
       </div>
     </footer>
@@ -87,6 +88,6 @@
 
 <div id="toasts"></div>
 
-<script src="app.js?v=4"></script>
+<script src="app.js?v=5"></script>
 </body>
 </html>