fix(nav): Back restores the focus after a click on the page's blank area

The page region is <main tabIndex=-1>, so a background click focused it and the
Back restore took that for the reader's choice. A resting place is marked and
one predicate (focusIsUnclaimed) decides "the focus is nowhere" everywhere.

frontend/src/components/DiagnosticsSettings.tsx

@@ -4,6 +4,7 @@ import { useTranslation } from "react-i18next";
 
 import { api, type DiagnosticsLevel } from "../lib/api";
 import { levelInForce } from "../lib/diagnostics";
+import { focusIsUnclaimed } from "../lib/useFocusKeeper";
 import { updateMeCache, useMe } from "../lib/useMe";
 import { Button } from "./ui/Button";
 import InlineNotice from "./ui/InlineNotice";
@@ -61,7 +62,7 @@ export default function DiagnosticsSettings() {
     const from = closedFrom.current;
     if (!from || from.isConnected) return;
     closedFrom.current = null;
-    if (document.activeElement && document.activeElement !== document.body) return;
+    if (!focusIsUnclaimed()) return;
     pickerRef.current?.querySelector<HTMLElement>('[aria-pressed="true"]')?.focus();
   });
   const watchFocus = () => {