Prefer image for queue task preview (#520)

Author: huchenlei

src/components/sidebar/tabs/queue/ResultItem.vue

@@ -52,6 +52,9 @@ onMounted(() => {
   aspect-ratio: 1 / 1;
   overflow: hidden;
   position: relative;
+  display: flex;
+  justify-content: center;
+  align-items: center;
 }
 
 .task-output-image {

src/components/sidebar/tabs/queue/TaskItem.vue

@@ -4,7 +4,7 @@
       <template v-if="task.displayStatus === TaskItemDisplayStatus.Completed">
         <ResultItem
           v-if="flatOutputs.length"
-          :result="flatOutputs[0]"
+          :result="task.previewOutput || flatOutputs[0]"
           @preview="handlePreview"
         />
       </template>