/* ========== 导航栏 (固定在顶部，铺满宽度) ========== */
header {
  background: #2c3e50;
  padding: 16px 0;
  box-shadow: 0 2px 8px rgba(0,0,0,0.1);
  position: fixed;
  top: 0;
  left: 0;
  width: 100%;
  z-index: 1000;
  transition: transform 0.3s ease; /* 平滑过渡 */
}

/* 隐藏状态：整体往上移出视口 */
header.hide {
  transform: translateY(-100%);
}

/* 给正文留出空间，避免被遮挡 */
body {
  padding-top: 70px; /* 根据 header 高度调整 */
}

nav ul {
  list-style: none;
  display: flex;
  gap: 32px;
  justify-content: center;
}

nav a {
  text-decoration: none;
  color: white;
  font-weight: 500;
  font-size: 18px;
  transition: color 0.2s;
}

nav a:hover {
  color: var(--primary-color);
}

/* ========== 页面三栏布局 ========== */
.layout {
  display: grid;
  grid-template-columns: 320px 1fr 320px;
  gap: 32px;
  max-width: 1440px;
  margin: 40px auto;
  padding: 0 24px;
  width: 100%;
  flex: 1;
}

@media (max-width: 1200px) {
  .layout {
    grid-template-columns: 300px 1fr;
    gap: 24px;
  }
}

@media (max-width: 768px) {
  .layout {
    grid-template-columns: 1fr;
    gap: 0;
    margin: 20px auto;
    padding: 0 16px;
  }
  .content {
    grid-column: 1 / -1;
    padding: 24px 16px;
    min-height: auto;
  }
}