fix(voice): activity tab rendered a permanently empty chart

VoiceActivityTimeline was never given a data prop — the Activity tab always
showed an empty Recharts bar chart while the connection tab already had live
speaker state. Replace the dead chart with a live speaker/activity list fed
from the same WebSocket data, so the tab reflects real state instead of
misleading empty bars.

services/frontend/src/app/(dashboard)/voice/page.tsx

@@ -148,7 +148,7 @@ export default function VoicePage() {
         </div>
       )}
 
-      {tab === "activity" && <VoiceActivityTimeline />}
+      {tab === "activity" && <VoiceActivityTimeline data={speakers} />}
     </div>
   );
 }