fix: 修复壁纸展示状态无法点击下载按钮的问题

src/components/Background.vue

@@ -1,5 +1,5 @@
 <template>
-  <div class="cover">
+  <div :class="store.backgroundShow ? 'cover show' : 'cover'">
     <img
       v-show="store.imgLoadStatus"
       class="bg"
@@ -99,6 +99,10 @@ onBeforeUnmount(() => {
   transition: 0.25s;
   z-index: -1;
 
+  &.show {
+    z-index: 1;
+  }
+
   .bg {
     position: absolute;
     left: 0;