fix(i18n): replace hardcoded VoiceAgent locale ternary with translation key

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

src/components/configurator/VoiceAgent.tsx

@@ -259,7 +259,7 @@ export default function VoiceAgent({ locale, onComplete }: VoiceAgentProps) {
             style={{ background: 'linear-gradient(135deg, #006494, #5BA4D9)' }}
           >
             <Mic size={16} />
-            {locale === 'fr' ? 'Démarrer la conversation' : 'Start Conversation'}
+            {t('voice.startConversation')}
           </button>
         )}