.portfolio-back {
  position: fixed;
  z-index: 999999;
  top: max(12px, env(safe-area-inset-top));
  left: max(12px, env(safe-area-inset-left));
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 42px;
  padding: 0 15px;
  border: 1px solid rgba(255, 255, 255, 0.3);
  border-radius: 999px;
  background: rgba(22, 22, 28, 0.82);
  color: #fff;
  box-shadow: 0 10px 26px rgba(0, 0, 0, 0.2);
  font: 700 13px/1 "Microsoft YaHei", "PingFang SC", system-ui, sans-serif;
  text-decoration: none;
  backdrop-filter: blur(12px);
}

.portfolio-back:hover,
.portfolio-back:focus-visible { background: rgba(22, 22, 28, 0.96); }

@media (max-width: 640px) {
  .portfolio-back { min-height: 40px; padding: 0 12px; font-size: 12px; }
}