feat(conversation): OpenAI gpt-realtime-2.1 라이브 음성 대화 Premium 백엔드

- realtime-token Edge Function: 티어 검증 + realtime_session 쿼터 + ephemeral key 발급
- useRealtimeConversation 훅: WebRTC 직결 (마이크 → OpenAI, 오디오 자동 재생)
- VoiceConversationPage: conversationBackend 분기 + 연결 실패 시 로컬 fallback
- AppConfig.conversationBackend + SettingsModal 음성 대화 엔진 선택
- ErrorCode 799 ConversationRealtimeTokenFailed, i18n ko/en

apps/desktop/src/renderer/components/SettingsModal.tsx

@@ -722,6 +722,37 @@ export function SettingsModal({ open, onClose }: SettingsModalProps): React.Reac
                   : t('settings.backend.localHint')}
               </Typography>
 
+              <Divider sx={{ borderColor: d3roPalette.border.subtle }} />
+
+              <Typography variant="overline" sx={{ color: d3roPalette.text.label, letterSpacing: '1.5px' }}>
+                {t('settings.conversationBackend')}
+              </Typography>
+
+              <FormControl size="small">
+                <InputLabel>{t('settings.conversationBackend')}</InputLabel>
+                <Select
+                  label={t('settings.conversationBackend')}
+                  value={config.conversationBackend ?? 'local'}
+                  onChange={(e) => {
+                    const next = e.target.value
+                    updateConfig('conversationBackend', next)
+                    if (next === 'realtime') {
+                      // Realtime은 로그인+구독 필요 — 라이선스 모달로 현재 티어 안내
+                      window.dispatchEvent(new Event('d3ro:open-license-modal'))
+                    }
+                  }}
+                >
+                  <MenuItem value="local">{t('settings.convBackend.local')}</MenuItem>
+                  <MenuItem value="realtime">{t('settings.convBackend.realtime')}</MenuItem>
+                </Select>
+              </FormControl>
+
+              <Typography variant="body2" sx={{ color: d3roPalette.text.inactive, fontSize: '11px' }}>
+                {config.conversationBackend === 'realtime'
+                  ? t('settings.convBackend.realtimeHint')
+                  : t('settings.convBackend.localHint')}
+              </Typography>
+
               {config.llmBackend !== 'premium' && (
                 <>
                   <Divider sx={{ borderColor: d3roPalette.border.subtle }} />