fix: end voice call when brief generation completes

The WebSocket, mic, and audio contexts were never torn down after
complete_brief succeeded, so the Gemini agent kept listening and
responding in the background. Now endConversation() fires as soon as
the brief arrives, and the Start button is hidden during the 1.5s
transition so it doesn't flash.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/configurator/VoiceAgent.tsx

@@ -73,17 +73,18 @@ export default function VoiceAgent({ locale, onComplete }: VoiceAgentProps) {
     }
   }, [transcript]);
 
-  // Handle completion
+  // Handle completion — end the call, then transition
   useEffect(() => {
     if (completedBrief && completedFormData) {
-      console.log('[VoiceAgent] Brief complete, transitioning in 1.5s...');
+      console.log('[VoiceAgent] Brief complete, ending conversation and transitioning in 1.5s...');
+      endConversation();
       const timer = setTimeout(() => {
         console.log('[VoiceAgent] Calling onComplete');
         onComplete(completedBrief, completedFormData);
       }, 1500);
       return () => clearTimeout(timer);
     }
-  }, [completedBrief, completedFormData, onComplete]);
+  }, [completedBrief, completedFormData, onComplete, endConversation]);
 
   // Orb animation driven by agent amplitude
   const amplitudeValue = useMotionValue(0);
@@ -255,7 +256,7 @@ export default function VoiceAgent({ locale, onComplete }: VoiceAgentProps) {
 
       {/* Controls */}
       <div className="flex items-center justify-center gap-3 pt-2">
-        {status === 'idle' && (
+        {status === 'idle' && !completedBrief && (
           <button
             type="button"
             onClick={startConversation}