fix: display timestamps in user's local timezone, not UTC

Two places in Dashboard used toISOString() for display, which always
renders UTC. Changed to toLocaleString() for the user's local timezone.

- Exchanges table "Started" column
- Detail panel "Timestamp" field

API query parameters correctly continue using toISOString() (UTC).

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

ui/src/pages/Dashboard/Dashboard.tsx

@@ -96,7 +96,7 @@ export default function Dashboard() {
     { key: 'routeId', header: 'Route', sortable: true, render: (v) => <span>{String(v)}</span> },
     { key: 'applicationName', header: 'Application', sortable: true, render: (v) => <span>{String(v ?? '')}</span> },
     { key: 'executionId', header: 'Exchange ID', sortable: true, render: (v) => <MonoText size="xs">{String(v)}</MonoText> },
-    { key: 'startTime', header: 'Started', sortable: true, render: (v) => <MonoText size="xs">{new Date(v as string).toISOString().replace('T', ' ').slice(0, 19)}</MonoText> },
+    { key: 'startTime', header: 'Started', sortable: true, render: (v) => <MonoText size="xs">{new Date(v as string).toLocaleString()}</MonoText> },
     {
       key: 'durationMs', header: 'Duration', sortable: true,
       render: (v) => <MonoText size="sm">{formatDuration(v as number)}</MonoText>,
@@ -219,7 +219,7 @@ export default function Dashboard() {
               </div>
               <div className={styles.overviewRow}>
                 <span className={styles.overviewLabel}>Timestamp</span>
-                <MonoText size="xs">{detail.startTime ? new Date(detail.startTime).toISOString().replace('T', ' ').slice(0, 19) : '—'}</MonoText>
+                <MonoText size="xs">{detail.startTime ? new Date(detail.startTime).toLocaleString() : '—'}</MonoText>
               </div>
             </div>
           </div>