Guard fold curtain resize observer

ui/src/components/FoldCurtain.tsx

@@ -80,6 +80,7 @@ export function FoldCurtain({
       setHasMeasured(true);
     };
     measure();
+    if (typeof ResizeObserver === "undefined") return;
     const observer = new ResizeObserver(measure);
     observer.observe(el);
     return () => observer.disconnect();