Fix sidebar cut-off on small screens in admin UI (#23764)

app/javascript/styles/mastodon/admin.scss

@@ -384,7 +384,7 @@ $content-width: 840px;
           position: fixed;
           z-index: 10;
           width: 100%;
-          height: calc(100vh - 56px);
+          height: calc(100% - 56px);
           left: 0;
           bottom: 0;
           overflow-y: auto;