/* 网格系统 */
.grid {
    display: grid;
    gap: var(--spacing-lg);
}

.grid-2 { grid-template-columns: repeat(2, 1fr); }
.grid-3 { grid-template-columns: repeat(3, 1fr); }
.grid-4 { grid-template-columns: repeat(4, 1fr); }

/* 弹性布局 */
.flex { display: flex; }
.flex-col { flex-direction: column; }
.items-center { align-items: center; }
.justify-between { justify-content: space-between; }

/* 定位系统 */
.sticky-top { position: sticky; top: 0; }
.fixed-bottom { position: fixed; bottom: 0; left: 0; right: 0; }