feat(merch/detail): add scroll hint for long product description

1. 新增商品详情长描述滚动提示组件，展开长描述时显示3秒
2. 调整描述文本容器底部内边距避免提示遮挡内容
3. 替换原有自动滚动动画为上下弹跳提示动画
4. 优化展开描述的逻辑，展开长文本时触发提示显示

pages_promotion/merch/detail.vue

@@ -24,7 +24,13 @@
 						<text class="icox icox-xing"></text>
 					</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="desc-text" :class="{ 'desc-expanded': isDescExpanded }" @click="toggleDesc">
+							<view>{{ merch.desc }}</view>
+							<view class="scroll-hint" v-if="isShowScrollHint">
+								<text class="scroll-icon">↓</text>
+								<text class="scroll-text">上滑查看更多</text>
+							</view>							
+						</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>
@@ -329,7 +335,8 @@ export default {
 			screenWidth: uni.getSystemInfoSync().windowWidth,
 			changeStyleDebounce: null,
 			headerHeightInited: false,
-			isDescExpanded: false
+			isDescExpanded: false,
+			isShowScrollHint: false
 		};
 	},
 	mixins: [list],
@@ -364,6 +371,12 @@ export default {
 		toggleDesc() {
 			if (!this.merch.desc) return;
 			this.isDescExpanded = !this.isDescExpanded;
+			if (this.isDescExpanded && this.merch.desc.length > 200) {
+				this.isShowScrollHint = true;
+				setTimeout(() => {
+					this.isShowScrollHint = false;
+				}, 3000);
+			}
 		},
 		initHeaderHeight() {
 			if (!this.merch || !this.merch.merch_name) return;
@@ -457,13 +470,37 @@ export default {
 	-webkit-box-orient: unset;
 	max-height: 300rpx;
 	word-break: break-all;
+	padding-bottom: 80rpx;
 }
 
-@keyframes autoScroll {
+.scroll-hint {
-	0% { transform: translateY(0); }
+	position: absolute;
-	30% { transform: translateY(-100rpx); }
+	bottom: 0;
-	60% { transform: translateY(-200rpx); }
+	left: 0;
-	100% { transform: translateY(0); }
+	right: 0;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	padding: 16rpx 0;
+	background: linear-gradient(to top, rgba(255,255,255,0.95), rgba(255,255,255,0.8));
+	z-index: 12;
+}
+
+.scroll-icon {
+	font-size: 28rpx;
+	color: #666;
+	margin-right: 8rpx;
+	animation: bounceDown 1s ease-in-out infinite;
+}
+
+.scroll-text {
+	font-size: 24rpx;
+	color: #666;
+}
+
+@keyframes bounceDown {
+	0%, 100% { transform: translateY(0); }
+	50% { transform: translateY(8rpx); }
 }
 
 .expand-tip {