fix: account menu being shown under the test config

closes monkeytypegame#6886

Author: Miodec

frontend/src/styles/nav.scss

@@ -241,7 +241,7 @@ header {
   display: grid;
   grid-auto-flow: column;
   grid-template-columns: auto 1fr;
-  z-index: 2;
+  z-index: 3;
   align-items: center;
   gap: 0.5rem;
   -webkit-user-select: none;