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/ui/PageScroller.tsx

@@ -1,6 +1,6 @@
 import { useCallback, useLayoutEffect, useRef, useState, type ReactNode } from "react";
 import { useScrollFade } from "../../lib/useScrollFade";
-import { useFocusKeeper } from "../../lib/useFocusKeeper";
+import { FOCUS_REST_ATTRS, useFocusKeeper } from "../../lib/useFocusKeeper";
 import { StickyTopProvider } from "./stickyTop";
 import { CONTENT_ID } from "./landmarks";
 import { ensureNavId, peekNavId, subscribeNav } from "../../lib/history";
@@ -158,11 +158,13 @@ export default function PageScroller({
 
   return (
     // `id` + `tabIndex={-1}`: the skip link's target (ui/SkipLink) takes the focus, so the next Tab
-    // lands on the page's first control.
+    // lands on the page's first control. A click on the page's background focuses it too, which is
+    // the focus RESTING, not a choice a Back restore must respect (`FOCUS_REST_ATTRS`).
     <main
       ref={setRef}
       id={CONTENT_ID}
       tabIndex={-1}
+      {...FOCUS_REST_ATTRS}
       style={fade.style}
       className="flex-1 min-w-0 overflow-y-auto scrollbar-gutter-stable focus:outline-none"
     >