Fix DatePicker popup flashing at top-left in Chromium

Latent bug: useEffect runs after paint, so the popup rendered at
{top:0, left:0} before repositioning. Switched to useLayoutEffect
which runs synchronously before paint, ensuring correct position
on first frame. Both Chromium and Firefox unaffected by the change.

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

frontend/src/components/ui/date-picker.tsx

@@ -160,7 +160,7 @@ const DatePicker = React.forwardRef<HTMLButtonElement, DatePickerProps>(
       });
     }, [mode, usesNativeInput]);
 
-    React.useEffect(() => {
+    React.useLayoutEffect(() => {
       if (!open) return;
       updatePosition();
       window.addEventListener('scroll', updatePosition, true);