message: Fix border radius when there is only one element
diff: border-radius: 0 0 var(--boxel-border-radius) var(--boxel-border-radius);
  }
  +.boxel-menu__item:only-child {
  + border-radius: var(--boxel-border-radius);
  +}
  +
  .boxel-menu__item > a {
  width: 100%;
  padding: var(--boxel-sp-xs) var(--boxel-sp);
  