style(coupon): 调整优惠券卡片样式高度和层级

- 增加主内容区高度以改善视觉平衡
- 微调背景和边框元素尺寸及定位
- 降低详情区域z-index避免层级冲突
- 调整详情区域上边距保持布局紧凑

src/pages/coupon/index.vue

@@ -223,7 +223,7 @@ const toggleExpand = (coupon) => {
     
     .coupon-main {
         width: 100%;
-        height: 180rpx;
+        height: 200rpx;
         display: flex;
         justify-content: center;
         align-items: center;
@@ -232,21 +232,22 @@ const toggleExpand = (coupon) => {
 
     .coupon-bg {
         width: 704rpx;
-        height: 180rpx;
+        height: 190rpx;
         position: absolute;
         z-index: 1;
     }
 
     .coupon-border {
         width: 660rpx;
-        height: 132rpx;
+        height: 158rpx;
         position: absolute;
         z-index: 2;
+        top: 20rpx;
     }
     
     .coupon-content {
         width: 660rpx;
-        height: 132rpx;
+        height: 146rpx;
         position: relative;
         z-index: 3;
         display: flex;
@@ -384,8 +385,8 @@ const toggleExpand = (coupon) => {
         padding: 20rpx 30rpx;
         box-sizing: border-box;
         position: relative;
-        z-index: 3;
-        margin-top: -10rpx;
+        z-index: 1;
+        margin-top: -20rpx;
         
         .detail-row {
             display: flex;