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/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}%` }}>