实现一个常见的加载动画（旋转 + 淡入淡出）

.spinner {
  width: 40px;
  height: 40px;
  border: 4px solid #f3f3f3;
  border-top: 4px solid #3498db;
  border-radius: 50%;
  animation: spin 1s linear infinite;
}

@keyframes spin {
  0%   { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}

.dots {
  display: flex;
  gap: 8px;
}
.dots span {
  width: 12px;
  height: 12px;
  border-radius: 50%;
  background: #3498db;
  animation: bounce 1.4s ease-in-out infinite;
}
.dots span:nth-child(2) { animation-delay: 0.2s; }
.dots span:nth-child(3) { animation-delay: 0.4s; }

@keyframes bounce {
  0%, 80%, 100% { transform: scale(0); opacity: 0.3; }
  40% { transform: scale(1); opacity: 1; }
}

.pulse {
  width: 40px;
  height: 40px;
  background: #3498db;
  border-radius: 50%;
  animation: pulse 1.5s ease-in-out infinite;
}

@keyframes pulse {
  0%   { transform: scale(0.8); opacity: 1; }
  50%  { transform: scale(1.2); opacity: 0.5; }
  100% { transform: scale(0.8); opacity: 1; }
}

.skeleton {
  background: linear-gradient(90deg, #f0f0f0 25%, #e0e0e0 50%, #f0f0f0 75%);
  background-size: 200% 100%;
  animation: shimmer 1.5s infinite;
  border-radius: 4px;
}

@keyframes shimmer {
  0%   { background-position: 200% 0; }
  100% { background-position: -200% 0; }
}

.fade-in-up {
  opacity: 0;
  transform: translateY(20px);
  animation: fadeInUp 0.6s ease forwards;
}

@keyframes fadeInUp {
  to {
    opacity: 1;
    transform: translateY(0);
  }
}

/* 逐个延迟出现 */
.item:nth-child(1) { animation-delay: 0.1s; }
.item:nth-child(2) { animation-delay: 0.2s; }
.item:nth-child(3) { animation-delay: 0.3s; }

@media (prefers-reduced-motion: reduce) {
  * {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}