fix: first exchange click doesn't highlight selected row

On first click, Dashboard was in non-split mode. The click set
selectedId locally then triggered split view, which remounted
Dashboard — losing the selectedId state.

Added activeExchangeId prop passed from ExchangesPage so the
selection survives the remount. Also syncs via useEffect when
parent changes selection (e.g. correlated exchange navigation).

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

ui/src/pages/Dashboard/Dashboard.tsx

@@ -1,4 +1,4 @@
-import { useState, useMemo, useCallback } from 'react'
+import { useState, useMemo, useCallback, useEffect } from 'react'
 import { useParams, useNavigate, useSearchParams } from 'react-router'
 import { AlertTriangle, X, Search, Footprints, RotateCcw } from 'lucide-react'
 import {
@@ -169,17 +169,23 @@ export interface SelectedExchange {
 
 interface DashboardProps {
   onExchangeSelect?: (exchange: SelectedExchange) => void;
+  activeExchangeId?: string;
 }
 
-export default function Dashboard({ onExchangeSelect }: DashboardProps = {}) {
+export default function Dashboard({ onExchangeSelect, activeExchangeId }: DashboardProps = {}) {
   const { appId, routeId } = useParams<{ appId: string; routeId: string }>()
   const navigate = useNavigate()
   const [searchParams, setSearchParams] = useSearchParams()
   const textFilter = searchParams.get('text') || undefined
-  const [selectedId, setSelectedId] = useState<string | undefined>()
+  const [selectedId, setSelectedId] = useState<string | undefined>(activeExchangeId)
   const [sortField, setSortField] = useState<string>('startTime')
   const [sortDir, setSortDir] = useState<'asc' | 'desc'>('desc')
 
+  // Sync selection from parent (survives remount when split view toggles)
+  useEffect(() => {
+    if (activeExchangeId !== undefined) setSelectedId(activeExchangeId);
+  }, [activeExchangeId]);
+
   const { timeRange, statusFilters } = useGlobalFilters()
   const timeFrom = timeRange.start.toISOString()
   const timeTo = timeRange.end.toISOString()

ui/src/pages/Exchanges/ExchangesPage.tsx

@@ -102,7 +102,7 @@ export default function ExchangesPage() {
   const showSplit = !!selected || !!scopedRouteId;
 
   if (!showSplit) {
-    return <Dashboard onExchangeSelect={handleExchangeSelect} />;
+    return <Dashboard onExchangeSelect={handleExchangeSelect} activeExchangeId={selected?.executionId} />;
   }
 
   // Determine what the right panel shows
@@ -113,7 +113,7 @@ export default function ExchangesPage() {
   return (
     <div ref={containerRef} className={styles.splitView}>
       <div className={styles.leftPanel} style={{ width: `${splitPercent}%` }}>
-        <Dashboard onExchangeSelect={handleExchangeSelect} />
+        <Dashboard onExchangeSelect={handleExchangeSelect} activeExchangeId={selected?.executionId} />
       </div>
       <div className={styles.splitter} onPointerDown={handleSplitterDown} />
       <div className={styles.rightPanel} style={{ width: `${100 - splitPercent}%` }}>