/* Botón flotante para volver desde cada estructura */
.volver {
  position: fixed;
  left: 1rem;
  bottom: 1rem;
  z-index: 100;
  padding: .6rem 1.2rem;
  border-radius: 999px;
  background: #3a3a3a;
  color: #fff;
  font: 600 .9rem/1 "Nunito Sans", sans-serif;
  text-decoration: none;
  box-shadow: 0 4px 12px rgba(0,0,0,.2);
  opacity: .85;
  transition: opacity .2s;
}
.volver:hover { opacity: 1; }