Fix loading spinner z-index

frontend/src/app/components/block-fees-graph/block-fees-graph.component.ts

@@ -23,7 +23,7 @@ import { StateService } from '../../services/state.service';
       position: absolute;
       top: 50%;
       left: calc(50% - 15px);
-      z-index: 100;
+      z-index: 99;
     }
   `],
   changeDetection: ChangeDetectionStrategy.OnPush,