Perf: defer Monaco mount until browser idle to reduce TBT

All editors were mounting simultaneously on initial render, causing
Monaco to block the main thread (TBT 1520ms). Now uses requestIdleCallback
(timeout 1.5s) to defer Monaco initialization until after first paint.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -37,6 +37,13 @@ function runWhenIdle(callback, timeout = 2000) {
 }
 
 function Editor({ height, ...props }) {
+  const [ready, setReady] = useState(false);
+  useEffect(() => {
+    const id = requestIdleCallback
+      ? requestIdleCallback(() => setReady(true), { timeout: 1500 })
+      : setTimeout(() => setReady(true), 300);
+    return () => (requestIdleCallback ? cancelIdleCallback(id) : clearTimeout(id));
+  }, []);
   const fallbackStyle = height ? { height } : undefined;
   return (
     <Suspense
@@ -51,7 +58,11 @@ function Editor({ height, ...props }) {
         </div>
       }
     >
-      <MonacoEditor height={height} {...props} />
+      {ready ? (
+        <MonacoEditor height={height} {...props} />
+      ) : (
+        <div className="editor-loading" style={fallbackStyle} aria-hidden="true" />
+      )}
     </Suspense>
   );
 }