/* ==========================================================================
   回到顶部组件 (BackTop)
   ========================================================================== */

.zuoekai-backtop {
  position: fixed;
  right: 40px;
  bottom: 40px;
  z-index: 1000;
  display: flex;
  align-items: center;
  justify-content: center;
  width: 44px;
  height: 44px;
  border-radius: 50%;
  background-color: var(--zuoekai-color-surface-panel);
  border: 1px solid var(--zuoekai-color-border);
  box-shadow: var(--zuoekai-shadow-lg);
  cursor: pointer;
  opacity: 0;
  visibility: hidden;
  transform: translateY(20px);
  transition: all 0.3s cubic-bezier(0.4, 0, 0.2, 1);
  color: var(--zuoekai-color-text-secondary);
}

.zuoekai-backtop:hover {
  background-color: var(--zuoekai-color-surface-hover);
  color: var(--zuoekai-color-brand-600);
  transform: translateY(0) scale(1.1);
  box-shadow: var(--zuoekai-shadow-xl);
}

.zuoekai-backtop--visible {
  opacity: 1;
  visibility: visible;
  transform: translateY(0);
}

.zuoekai-backtop svg {
  width: 20px;
  height: 20px;
  stroke-width: 2.5;
}

/* --- 变体 (Variants) --- */

/* 1. 方形 (Square) */
.zuoekai-backtop--square {
  border-radius: var(--zuoekai-radius-md);
}

/* 2. 品牌色 (Brand) */
.zuoekai-backtop--primary {
  background-color: var(--zuoekai-color-brand-600);
  color: white;
  border: none;
}

.zuoekai-backtop--primary:hover {
  background-color: var(--zuoekai-color-brand-700);
  color: white;
}

/* --- 移动端适配 --- */
@media (max-width: 768px) {
  .zuoekai-backtop {
    right: 20px;
    bottom: 80px; /* Avoid overlapping bottom nav/dock */
    width: 40px;
    height: 40px;
  }
}