refactor(merch detail): extract search placeholder to lang config

extract the hardcoded search input placeholder into internationalization language config, unify the text with the existing common lang entry

pages_promotion/merch/detail.vue

@@ -4,7 +4,7 @@
 			<!-- 搜索区域 -->
 			<view class="search-wrap uni-flex uni-row" style="margin-bottom: 20rpx;">
 				<view class="flex-item input-wrap">
-					<input class="uni-input" maxlength="50" v-model="keyword" @confirm="search()" placeholder="请输入您要搜索的商品" />
+					<input class="uni-input" maxlength="50" v-model="keyword" @confirm="search()" :placeholder="$lang('merch.searchGoodsPlaceholder')" />
 					<text class="iconfont icon-sousuo3" @click.stop="search()"></text>
 				</view>
 				<view class="iconfont" :class="{ 'icon-apps': isList, 'icon-list': !isList }" @click="changeListStyle()"></view>