fix: 优化购物车移动端布局

- 商品项布局优化：复选框+图片+信息+小计在一行
- 数量选择器和删除按钮在下方单独一行
- 隐藏单价列，简化移动端显示
- 调整间距和字体大小

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/views/user/Cart.vue

@@ -933,6 +933,8 @@ onMounted(() => {
 @media (max-width: 768px) {
   .cart-page {
     padding-bottom: 20px;
+    padding-left: 12px;
+    padding-right: 12px;
   }
 
   .cart-header {
@@ -941,23 +943,56 @@ onMounted(() => {
 
   .cart-item {
     flex-wrap: wrap;
-    gap: 12px;
-    padding: 14px;
+    gap: 8px;
+    padding: 12px;
+    align-items: flex-start;
 
-    > .el-checkbox { width: 24px; order: 1; }
-    > .item-product { flex: 1; min-width: 0; order: 2; }
-    > .item-price { width: auto; order: 4; margin-left: 36px; }
-    > .item-quantity { width: auto; order: 5; }
-    > .item-subtotal { width: auto; order: 3; margin-left: auto; }
-    > .item-action { width: auto; order: 6; }
+    > .el-checkbox {
+      width: 24px;
+      order: 1;
+      margin-top: 24px;
+    }
+    > .item-product {
+      flex: 1;
+      min-width: 0;
+      order: 2;
+      width: calc(100% - 36px);
+    }
+    > .item-price {
+      display: none;
+    }
+    > .item-quantity {
+      order: 4;
+      width: 100%;
+      display: flex;
+      flex-direction: row;
+      align-items: center;
+      justify-content: flex-start;
+      gap: 12px;
+      padding-left: 36px;
+      margin-top: 8px;
+    }
+    > .item-subtotal {
+      order: 3;
+      width: auto;
+      margin-left: auto;
+      padding-right: 8px;
+    }
+    > .item-action {
+      order: 5;
+      width: 100%;
+      padding-left: 36px;
+      margin-top: 4px;
+    }
   }
 
   .item-product {
-    gap: 12px;
+    gap: 10px;
+    flex-wrap: nowrap;
 
     .item-image {
-      width: 80px;
-      height: 80px;
+      width: 70px;
+      height: 70px;
       flex-shrink: 0;
     }
   }
@@ -965,6 +1000,31 @@ onMounted(() => {
   .item-info {
     flex: 1;
     min-width: 0;
+    display: flex;
+    flex-direction: column;
+    justify-content: center;
+    gap: 4px;
+  }
+
+  .item-name {
+    font-size: 14px;
+    line-height: 1.4;
+  }
+
+  .item-quantity {
+    :deep(.el-input-number) {
+      width: 100px;
+    }
+
+    .stock-tip {
+      font-size: 11px;
+    }
+  }
+
+  .item-subtotal {
+    .subtotal-price {
+      font-size: 16px;
+    }
   }
 
   .payment-methods {
@@ -972,25 +1032,33 @@ onMounted(() => {
   }
 
   .cart-left {
-    gap: 16px;
+    gap: 12px;
   }
 
   .checkout-section {
-    padding: 16px;
+    padding: 12px;
   }
 
   .cart-summary-panel {
-    padding: 16px;
+    padding: 12px;
   }
 
   .summary-title {
     font-size: 16px;
-    margin-bottom: 16px;
-    padding-bottom: 12px;
+    margin-bottom: 12px;
+    padding-bottom: 10px;
   }
 
   .summary-total .total-amount {
     font-size: 20px;
   }
+
+  .address-item {
+    padding: 12px;
+  }
+
+  .payment-item {
+    padding: 12px 8px;
+  }
 }
 </style>