Fix loading spinner z-index

frontend/src/app/components/mempool-graph/mempool-graph.component.ts

@@ -18,7 +18,7 @@ import { download, formatterXAxis, formatterXAxisLabel } from '../../shared/grap
       position: absolute;
       top: 50%;
       left: calc(50% - 16px);
-      z-index: 100;
+      z-index: 99;
     }
   `],
   changeDetection: ChangeDetectionStrategy.OnPush,