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/Dashboard/Dashboard.tsx

@@ -114,7 +114,7 @@ export default function Dashboard() {
       <div className={styles.healthStrip}>
         <StatCard
           label="Exchanges"
-          value={totalCount.toLocaleString('en-US')}
+          value={totalCount.toLocaleString()}
           detail={`${successRate.toFixed(1)}% success rate`}
           trend={exchangeTrend > 0 ? 'up' : exchangeTrend < 0 ? 'down' : 'neutral'}
           trendValue={exchangeTrend > 0 ? `+${exchangeTrend.toFixed(0)}%` : `${exchangeTrend.toFixed(0)}%`}
@@ -124,7 +124,7 @@ export default function Dashboard() {
         <StatCard
           label="Success Rate"
           value={`${successRate.toFixed(1)}%`}
-          detail={`${(totalCount - failedCount).toLocaleString('en-US')} ok / ${failedCount} error`}
+          detail={`${(totalCount - failedCount).toLocaleString()} ok / ${failedCount} error`}
           trend={successRateDelta >= 0 ? 'up' : 'down'}
           trendValue={`${successRateDelta >= 0 ? '+' : ''}${successRateDelta.toFixed(1)}%`}
           accent="success"
@@ -147,8 +147,8 @@ export default function Dashboard() {
         />
         <StatCard
           label="Latency p99"
-          value={(stats?.p99LatencyMs ?? 0).toLocaleString('en-US')}
-          detail={`${(stats?.p99LatencyMs ?? 0).toLocaleString('en-US')}ms`}
+          value={(stats?.p99LatencyMs ?? 0).toLocaleString()}
+          detail={`${(stats?.p99LatencyMs ?? 0).toLocaleString()}ms`}
           sparkline={sparkLatency}
           accent="warning"
         />

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}>