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/IssueComposer.tsx

@@ -19,6 +19,7 @@ import {
 import { moduleDef } from "../lib/modules";
 import type { Page } from "../lib/urlState";
 import { qk } from "../lib/queryKeys";
+import { focusIsUnclaimed } from "../lib/useFocusKeeper";
 import { LEVEL_TAG, logClock } from "./logLineParts";
 import { Button } from "./ui/Button";
 import { IconButton } from "./ui/IconButton";
@@ -54,7 +55,7 @@ export default function IssueComposer({ id }: { id: string }) {
   useEffect(
     () => () => {
       window.setTimeout(() => {
-        if (document.activeElement === document.body)
+        if (focusIsUnclaimed())
           document.querySelector<HTMLElement>('nav [aria-current="page"]')?.focus();
       }, 0);
     },