feat: syntax-highlight decision trace JSON on order detail page

Add lightweight JSON highlighter for the decision trace panel:
- Keys: cyan
- Strings: green
- Numbers: yellow
- Booleans: purple
- Null: red
- Structural chars: gray

SQL explorer already uses Monaco with SQL highlighting — no changes needed there.

frontend/src/pages/OrderDetail.tsx

@@ -2,6 +2,55 @@ import { useParams } from '@tanstack/react-router';
 import { useOrder } from '../api/hooks';
 import { StatusBadge, LoadingSpinner, Card } from '../components/ui';
 
+/**
+ * Lightweight JSON syntax highlighter for read-only display.
+ * Returns React elements with colored spans for keys, strings, numbers, booleans, and null.
+ */
+function highlightJson(json: string): React.ReactNode {
+  const parts: React.ReactNode[] = [];
+  // Regex matches JSON tokens: strings, numbers, booleans, null, and structural chars
+  const tokenRe = /("(?:\\.|[^"\\])*")\s*:|("(?:\\.|[^"\\])*")|(-?\d+(?:\.\d+)?(?:[eE][+-]?\d+)?)|(\btrue\b|\bfalse\b)|(\bnull\b)|([{}[\],])/g;
+  let lastIndex = 0;
+  let match: RegExpExecArray | null;
+
+  while ((match = tokenRe.exec(json)) !== null) {
+    // Add any whitespace/text between tokens
+    if (match.index > lastIndex) {
+      parts.push(json.slice(lastIndex, match.index));
+    }
+
+    if (match[1]) {
+      // Key (string followed by colon)
+      parts.push(<span key={match.index} className="text-cyan-400">{match[1]}</span>);
+      parts.push(':');
+    } else if (match[2]) {
+      // String value
+      parts.push(<span key={match.index} className="text-green-400">{match[2]}</span>);
+    } else if (match[3]) {
+      // Number
+      parts.push(<span key={match.index} className="text-yellow-300">{match[3]}</span>);
+    } else if (match[4]) {
+      // Boolean
+      parts.push(<span key={match.index} className="text-purple-400">{match[4]}</span>);
+    } else if (match[5]) {
+      // Null
+      parts.push(<span key={match.index} className="text-red-400">{match[5]}</span>);
+    } else if (match[6]) {
+      // Structural characters
+      parts.push(<span key={match.index} className="text-gray-500">{match[6]}</span>);
+    }
+
+    lastIndex = match.index + match[0].length;
+  }
+
+  // Remaining text
+  if (lastIndex < json.length) {
+    parts.push(json.slice(lastIndex));
+  }
+
+  return <>{parts}</>;
+}
+
 export function OrderDetailPage() {
   const { id } = useParams({ from: '/orders/$id' });
   const { data: order, isLoading } = useOrder(id);
@@ -33,8 +82,8 @@ export function OrderDetailPage() {
       {order.decision_trace && Object.keys(order.decision_trace).length > 0 && (
         <Card>
           <h2 className="mb-2 text-sm font-medium text-gray-400">Decision Trace</h2>
-          <pre className="overflow-x-auto rounded bg-surface-950 p-3 text-xs text-gray-300">
-            {JSON.stringify(order.decision_trace, null, 2)}
+          <pre className="overflow-x-auto rounded bg-surface-950 p-3 text-xs leading-relaxed">
+            {highlightJson(JSON.stringify(order.decision_trace, null, 2))}
           </pre>
         </Card>
       )}