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

voice-poc/public/style.css

@@ -87,6 +87,7 @@ body {
   background: var(--red); transform: rotate(-40deg); border-radius: 1px;
 }
 .ctl:disabled { opacity: .35; cursor: not-allowed; }
+.ctl.active { color: var(--green); }
 
 /* main */
 #main { flex: 1; display: flex; flex-direction: column; min-width: 0; }