媒体元素若没有封面采用视频第一帧，提供后退方案

src/modules/report/SchemeMedia.vue

@@ -55,6 +55,8 @@ function onPlay(event?: Event) {
     <div class="flex-none mx-2" v-for="item in media" :key="item.title">
       <div v-if="item.url" class="relative h-32 rounded-lg	overflow-hidden" @click="handle(item)">
         <img class="size-full object-scale-down" v-if="item.poster" :src="item.poster" :alt="item.title" />
+        <video class="size-full object-scale-down" v-else-if="item.type === 'video'" :src="item.url" preload="metadata"></video>
+        <div class="size-full object-scale-down w-64 bg-gray-600" v-else></div>
         <van-icon class="play" v-if="item.type === 'video'" name="play-circle-o" />
       </div>
       <template v-if="item.title">