/* ========== 返回顶部按钮样式 ========== */
.back-to-top-btn {
  position: fixed;
  bottom: 30px;
  right: 30px;
  width: 48px;
  height: 48px;
  background: #333;
  color: #fff;
  border-radius: 50%;
  text-align: center;
  line-height: 48px;
  font-size: 24px;
  cursor: pointer;
  opacity: 0;
  visibility: hidden;
  transform: translateY(20px);
  transition: all 0.3s ease;
  z-index: 9999;
  box-shadow: 0 4px 12px rgba(0,0,0,0.25);
}

.back-to-top-btn.show {
  opacity: 1;
  visibility: visible;
  transform: translateY(0);
}

.back-to-top-btn:hover {
  background: #555;
  box-shadow: 0 6px 16px rgba(0,0,0,0.3);
}

/* 移动端适配 */
@media screen and (max-width: 768px) {
  .back-to-top-btn {
    width: 42px;
    height: 42px;
    line-height: 42px;
    font-size: 20px;
    bottom: 20px;
    right: 20px;
  }
}