style:预览蒙层样式优化

src/modules/course-manage/offline-course/modal/PreviewOfflineModal.less

@@ -20,11 +20,10 @@
       .cover {
         position: absolute;
         width: 100%;
-        height: 32px;
+        height: 36px;
         left: 0;
         bottom: 0;
-        background-color: rgba(0, 0, 0, 0.05);
-        z-index: 10;
+        background: linear-gradient(180deg, rgba(0, 0, 0, 0) 0%, rgba(0, 0, 0, 0.5) 100%);        z-index: 10;
       }
       .course-cover, .course-url {
         width: 100%;
@@ -37,6 +36,7 @@
         left: 16px;
         font-size: 10px;
         color: #fff;
+        z-index: 10;
       }
       .cover-state {
         position: absolute;
@@ -48,6 +48,7 @@
         padding: 0 8px;
         color: #fff;
         border-radius: 9px;
+        z-index: 10;
       }
     }