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/lib/listNav.ts

@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState, type RefObject } from "react";
 import { dispatch, useKeyScope } from "./keymap";
 import { scrollParent } from "./listPaging";
-import { isStandInFocus } from "./useFocusKeeper";
+import { focusIsUnclaimed } from "./useFocusKeeper";
 
 // ── Keyboard navigation over lists and grids (E11 S4, UX-6) ──────────────────────────────────────
 // Before this, a feed card cost ~10 Tab stops (thumb, play, info, title, channel, five actions), so
@@ -148,6 +148,9 @@ function onFocusIn(e: FocusEvent): void {
   const list =
     item && lists.get(item.parentElement?.closest<HTMLElement>(`[${LIST}]`) as HTMLElement);
   if (item && list) last = { list, index: indexOf(item), el: item };
+  // The focus coming to REST on the page region (a click on its background) is not leaving the list
+  // for other content — like a click on <body>, which fires no focusin at all.
+  else if (focusIsUnclaimed(target)) return;
   // Focus went on to page content that is not an item — the search box, a filter chip: the reader
   // left the list, and whatever it shows next (new results) is not "the slot they were in" (deep-review
   // E11 round 2). A layer opened FROM the item — its menu, a confirm, the Undo toast its removal put
@@ -375,8 +378,7 @@ function refocusSlot(list: NavList, index: number, removed: HTMLElement): void {
       if (++frames < REMOVE_FRAMES) requestAnimationFrame(wait);
       return;
     }
-    const active = document.activeElement;
-    if (active && active !== document.body && !isStandInFocus(active)) return;
+    if (!focusIsUnclaimed()) return;
     const n = list.count();
     if (n > 0) focusIndex(list, Math.min(index, n - 1));
   };
@@ -450,8 +452,7 @@ export function restoreNavPosition(pos: NavPosition): () => void {
   const deadline = Date.now() + RESTORE_MS;
   let timer: ReturnType<typeof setTimeout> | undefined;
   const attempt = () => {
-    const active = document.activeElement;
-    if (active && active !== document.body && !isStandInFocus(active)) return;
+    if (!focusIsUnclaimed()) return;
     const all = ordered();
     const list = pos.label
       ? all.find((l) => labelOf(l) === pos.label)