fix: 重构购物车移动端商品布局

- 复选框左上角绝对定位
- 删除按钮右上角绝对定位
- 商品图片+信息单独一行
- 数量选择器单独一行
- 小计金额单独一行显示

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

frontend/src/views/user/Cart.vue

@@ -944,40 +944,19 @@ onMounted(() => {
   .cart-item {
     display: block;
     padding: 12px;
+    padding-left: 40px;
     position: relative;
   }
 
+  /* 复选框 - 左上角 */
   .cart-item > .el-checkbox {
     position: absolute;
-    top: 12px;
+    top: 14px;
     left: 12px;
     width: 24px;
   }
 
-  .cart-item > .item-product {
-    display: flex;
-    gap: 10px;
-    padding-left: 32px;
-    padding-right: 60px;
-  }
-
-  .cart-item > .item-price {
-    display: none;
-  }
-
-  .cart-item > .item-quantity {
-    display: flex;
-    align-items: center;
-    justify-content: space-between;
-    margin-top: 10px;
-    padding-left: 32px;
-  }
-
-  .cart-item > .item-subtotal {
-    text-align: right;
-    margin-top: 8px;
-  }
-
+  /* 删除按钮 - 右上角 */
   .cart-item > .item-action {
     position: absolute;
     top: 12px;
@@ -985,6 +964,32 @@ onMounted(() => {
     width: auto;
   }
 
+  /* 商品信息 - 第一行 */
+  .cart-item > .item-product {
+    display: flex;
+    gap: 10px;
+    padding-right: 50px;
+    margin-bottom: 10px;
+  }
+
+  /* 隐藏单价列 */
+  .cart-item > .item-price {
+    display: none;
+  }
+
+  /* 数量 - 单独一行 */
+  .cart-item > .item-quantity {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    margin-bottom: 8px;
+  }
+
+  /* 小计金额 - 单独一行 */
+  .cart-item > .item-subtotal {
+    text-align: left;
+  }
+
   .item-product .item-image {
     width: 50px;
     height: 50px;
@@ -1002,6 +1007,7 @@ onMounted(() => {
 
   .item-subtotal .subtotal-price {
     font-size: 16px;
+    color: #f56c6c;
   }
 
   .payment-methods {