/* Back to top */
.back-to-top {
  position: fixed;
  right: var(--space-md);
  bottom: var(--space-md);
  width: 2.8rem;
  height: 2.8rem;
  border-radius: 50%;
  background: var(--flame);
  color: var(--space);
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: 0 0.4rem 1rem rgba(0,0,0,0.35);
  opacity: 0;
  visibility: hidden;
  transform: translateY(0.5rem);
  transition: opacity 0.2s ease, transform 0.2s ease, visibility 0.2s;
  z-index: 90;
}
.back-to-top.is-visible { opacity: 1; visibility: visible; transform: translateY(0); }