fix: 重新设计购物车移动端为一行布局

- 所有元素在一行横向排列
- 复选框 | 图片60px | 商品名称 | 数量 | 小计 | 删除
- 移除绝对定位，使用自然流布局
- 隐藏库存提示和标签简化显示

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

frontend/src/views/user/Cart.vue

@@ -946,42 +946,31 @@ onMounted(() => {
     overflow-y: visible;
   }
 
-  /* 商品项容器 */
+  /* 商品项容器 - 两列布局 */
   .cart-item {
     display: flex !important;
-    flex-direction: column !important;
+    flex-direction: row !important;
     flex-wrap: nowrap !important;
-    padding: 10px !important;
-    padding-top: 36px !important;
+    align-items: flex-start !important;
+    padding: 12px !important;
     position: relative !important;
-    gap: 8px;
+    gap: 10px !important;
   }
 
-  /* 复选框 - 左上角 */
+  /* 复选框 - 左侧 */
   .cart-item > .el-checkbox {
-    position: absolute !important;
-    top: 10px !important;
-    left: 10px !important;
-    width: 24px !important;
-    z-index: 10 !important;
+    width: 32px !important;
+    flex-shrink: 0 !important;
+    align-self: center !important;
+    margin-right: 0 !important;
   }
 
-  /* 删除按钮 - 右上角 */
-  .cart-item > .item-action {
-    position: absolute !important;
-    top: 10px !important;
-    right: 10px !important;
-    width: auto !important;
-    z-index: 10 !important;
-  }
-
-  /* 商品信息行 */
+  /* 商品图片+信息 - 中间主体 */
   .cart-item > .item-product {
     display: flex !important;
     flex-direction: row !important;
-    width: 100% !important;
-    padding-left: 28px !important;
-    margin-bottom: 8px !important;
+    flex: 1 !important;
+    min-width: 0 !important;
     cursor: pointer;
   }
 
@@ -990,55 +979,70 @@ onMounted(() => {
     display: none !important;
   }
 
-  /* 数量行 */
+  /* 数量 - 商品信息下方 */
   .cart-item > .item-quantity {
-    width: 100% !important;
-    flex-direction: row !important;
-    align-items: center !important;
-    justify-content: flex-start !important;
-    padding-left: 28px !important;
-    margin-bottom: 4px !important;
+    width: auto !important;
+    flex-shrink: 0 !important;
+    align-self: center !important;
+    margin-left: 8px !important;
   }
 
-  /* 小计行 */
+  /* 小计 - 右侧 */
   .cart-item > .item-subtotal {
-    width: 100% !important;
-    text-align: left !important;
-    padding-left: 28px !important;
+    width: auto !important;
+    flex-shrink: 0 !important;
+    align-self: center !important;
+    text-align: right !important;
+    margin-left: 8px !important;
+  }
+
+  /* 删除按钮 - 最右侧 */
+  .cart-item > .item-action {
+    width: auto !important;
+    flex-shrink: 0 !important;
+    align-self: center !important;
   }
 
   .item-product .item-image {
-    width: 50px !important;
-    height: 50px !important;
+    width: 60px !important;
+    height: 60px !important;
+    margin-right: 10px !important;
   }
 
   .item-info {
     flex: 1;
     min-width: 0;
-    gap: 4px !important;
+    display: flex;
+    flex-direction: column;
+    gap: 6px;
   }
 
   .item-name {
-    font-size: 13px !important;
-    -webkit-line-clamp: 1 !important;
+    font-size: 14px !important;
+    line-height: 1.3 !important;
+    -webkit-line-clamp: 2 !important;
   }
 
   .item-tags {
-    display: none;
+    display: none !important;
+  }
+
+  .item-off {
+    font-size: 12px !important;
   }
 
   .item-quantity :deep(.el-input-number) {
-    width: 90px;
+    width: 80px !important;
   }
 
   .item-quantity .stock-tip {
-    font-size: 11px;
+    display: none !important;
   }
 
   .item-subtotal .subtotal-price {
-    font-size: 15px;
-    font-weight: 600;
-    color: #f56c6c;
+    font-size: 15px !important;
+    font-weight: 600 !important;
+    color: #f56c6c !important;
   }
 
   /* 支付方式 */