feat(merch/detail): 优化商品描述展示逻辑

1. 替换原有长按弹窗展示为内嵌展开收起功能
2. 长描述支持自动滚动动画
3. 新增展开收起提示按钮
4. 调整相关布局样式与层级

pages_promotion/merch/detail.vue

@@ -23,7 +23,12 @@
 						<text class="icox icox-xing"></text>
 						<text class="icox icox-xing"></text>
 					</view>
-					<view class="desc-text" @longpress="showFullDesc">{{ merch.desc }}</view>
+					<view class="desc-wrap">
+						<view class="desc-text" :class="{ 'desc-expanded': isDescExpanded }" :style="{ animation: isDescExpanded && merch.desc && merch.desc.length > 200 ? 'autoScroll 2s ease-in-out' : '' }" @click="toggleDesc">{{ merch.desc }}</view>
+						<view class="expand-tip" v-if="merch.desc && merch.desc.length > 50" @click.stop="toggleDesc">
+							<text class="hint-arrow">{{ isDescExpanded ? '↑ 收起' : '↓ 展开' }}</text>
+						</view>
+					</view>
 				</view>
 			</view>
 
@@ -323,7 +328,8 @@ export default {
 			headerHeight: 320,
 			screenWidth: uni.getSystemInfoSync().windowWidth,
 			changeStyleDebounce: null,
-			headerHeightInited: false
+			headerHeightInited: false,
+			isDescExpanded: false
 		};
 	},
 	mixins: [list],
@@ -346,15 +352,19 @@ export default {
 				}
 			});
 		},
-		showFullDesc() {
+		toggleDesc() {
 			if (this.merch.desc) {
-				uni.showModal({
-					title: '商家描述',
-					content: this.merch.desc,
-					showCancel: false
-				});
+				this.isDescExpanded = !this.isDescExpanded;
 			}
 		},
+		handleDescClick() {
+			if (!this.merch.desc) return;
+			this.isDescExpanded = !this.isDescExpanded;
+		},
+		toggleDesc() {
+			if (!this.merch.desc) return;
+			this.isDescExpanded = !this.isDescExpanded;
+		},
 		initHeaderHeight() {
 			if (!this.merch || !this.merch.merch_name) return;
 			if (this.headerHeightInited) return;
@@ -388,6 +398,7 @@ export default {
 	/* #ifndef H5 */
 	top: 0;
 	/* #endif */
+	z-index: 2;
 }
 
 .merch-image-wrap {
@@ -398,11 +409,15 @@ export default {
 .merch-info {
 	margin-left: 20rpx;
 	padding-top: 22rpx;
+	position: relative;
 }
 
 .merch-info-wrap {
 	display: flex;
 	padding: 20rpx;
+	position: relative;
+	z-index: 10;
+	background: #fff;
 }
 
 .merch-name {
@@ -415,12 +430,56 @@ export default {
 	color: rgb(255 202 40);
 }
 
+.desc-wrap {
+	display: flex;
+	flex-wrap: nowrap;
+	align-items: flex-end;
+	position: relative;
+}
+
 .desc-text {
+	flex: 1;
+	min-width: 0;
 	overflow: hidden;
 	text-overflow: ellipsis;
 	display: -webkit-box;
 	-webkit-line-clamp: 2;
 	-webkit-box-orient: vertical;
+	word-break: break-all;
+}
+
+.desc-expanded {
+	position: relative;
+	overflow-y: auto;
+	text-overflow: inherit;
+	display: block;
+	-webkit-line-clamp: unset;
+	-webkit-box-orient: unset;
+	max-height: 300rpx;
+	word-break: break-all;
+}
+
+@keyframes autoScroll {
+	0% { transform: translateY(0); }
+	30% { transform: translateY(-100rpx); }
+	60% { transform: translateY(-200rpx); }
+	100% { transform: translateY(0); }
+}
+
+.expand-tip {
+	flex-shrink: 0;
+	margin-left: 10rpx;
+	z-index: 11;
+}
+
+.hint-arrow {
+	display: inline-block;
+	padding: 8rpx 16rpx;
+	background: rgba(255, 255, 255, 0.95);
+	border-radius: 20rpx;
+	font-size: 24rpx;
+	color: #666;
+	box-shadow: 0 2rpx 10rpx rgba(0, 0, 0, 0.1);
 }
 
 ::v-deep .uni-tag--primary.uni-tag--inverted {