@media (max-width: 900px) {
  body { overflow-x: hidden; }
  .shell { display: block; min-width: 0; max-width: 100vw; }
  .sidebar, .content { min-width: 0; max-width: 100vw; }
  .sidebar nav { max-width: calc(100vw - 16px); overflow-x: auto; }
}