fix: 修复购物车移动端删除按钮位置

- 删除按钮绝对定位在右上角
- 复选框绝对定位在左上角
- 商品信息占整行
- 数量选择器单独一行显示

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

frontend/src/views/user/Cart.vue

@@ -942,90 +942,67 @@ onMounted(() => {
   }
 
   .cart-item {
-    flex-wrap: wrap;
+    display: block;
-    gap: 8px;
     padding: 12px;
-    align-items: flex-start;
+    position: relative;
+  }
 
-    > .el-checkbox {
+  .cart-item > .el-checkbox {
+    position: absolute;
+    top: 12px;
+    left: 12px;
     width: 24px;
-      order: 1;
-      margin-top: 24px;
   }
-    > .item-product {
+
-      flex: 1;
+  .cart-item > .item-product {
-      min-width: 0;
+    display: flex;
-      order: 2;
+    gap: 10px;
-      width: calc(100% - 36px);
+    padding-left: 32px;
+    padding-right: 60px;
   }
-    > .item-price {
+
+  .cart-item > .item-price {
     display: none;
   }
-    > .item-quantity {
+
-      order: 4;
+  .cart-item > .item-quantity {
-      width: 100%;
     display: flex;
-      flex-direction: row;
     align-items: center;
-      justify-content: flex-start;
+    justify-content: space-between;
-      gap: 12px;
+    margin-top: 10px;
-      padding-left: 36px;
+    padding-left: 32px;
+  }
+
+  .cart-item > .item-subtotal {
+    text-align: right;
     margin-top: 8px;
   }
-    > .item-subtotal {
+
-      order: 3;
+  .cart-item > .item-action {
+    position: absolute;
+    top: 12px;
+    right: 12px;
     width: auto;
-      margin-left: auto;
-      padding-right: 8px;
-    }
-    > .item-action {
-      order: 5;
-      width: 100%;
-      padding-left: 36px;
-      margin-top: 4px;
-    }
   }
 
-  .item-product {
+  .item-product .item-image {
-    gap: 10px;
-    flex-wrap: nowrap;
-
-    .item-image {
     width: 70px;
     height: 70px;
     flex-shrink: 0;
   }
-  }
 
   .item-info {
     flex: 1;
     min-width: 0;
-    display: flex;
-    flex-direction: column;
-    justify-content: center;
-    gap: 4px;
   }
 
-  .item-name {
+  .item-quantity :deep(.el-input-number) {
-    font-size: 14px;
-    line-height: 1.4;
-  }
-
-  .item-quantity {
-    :deep(.el-input-number) {
     width: 100px;
   }
 
-    .stock-tip {
+  .item-subtotal .subtotal-price {
-      font-size: 11px;
-    }
-  }
-
-  .item-subtotal {
-    .subtotal-price {
     font-size: 16px;
   }
-  }
 
   .payment-methods {
     grid-template-columns: repeat(2, 1fr);