fix: remove hardcoded locales from UI formatting

Use browser default locale instead of hardcoded 'en-US' and 'en-GB'
for number and time formatting.

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

ui/src/pages/ExchangeDetail/ExchangeDetail.tsx

@@ -131,7 +131,7 @@ export default function ExchangeDetail() {
             <div className={styles.headerStat}>
               <div className={styles.headerStatLabel}>Started</div>
               <div className={styles.headerStatValue}>
-                {detail.startTime ? new Date(detail.startTime).toLocaleTimeString('en-GB', { hour: '2-digit', minute: '2-digit', second: '2-digit' }) : '—'}
+                {detail.startTime ? new Date(detail.startTime).toLocaleTimeString([], { hour: '2-digit', minute: '2-digit', second: '2-digit' }) : '—'}
               </div>
             </div>
             <div className={styles.headerStat}>