fix(ui): Back finds the item again past the focus keeper; nested drags stay in their list

Deep review of the debt burn, each measured in Chrome:
- Back from a sub-view whose focused control the swap removed: the focus keeper put the focus on a
  stand-in (the returning list's first item) and `restoreNavPosition`, finding the focus "already
  somewhere", gave up — Plex show → Back landed on tile 0, not the tile it was opened from. A
  stand-in the keeper placed now counts as untouched, like <body>. The keeper's walk up also stops
  at its container: a slot left with nothing focusable no longer sends the focus to the first
  control of the whole document.
- DataTable enabled both layouts' keyboard lists with the same flag, and `useListNav` reads its ref
  only when the flag changes: after a resize across `md`, the layout that mounted was never
  registered (no j/k) and the one that left stayed registered. Each is enabled with its layout.
- A Plex playlist nests blocks and each show's episodes in one DndContext; closestCenter and the
  keyboard coordinates weighed every droppable, so a movie landed "over" an episode — announced as
  a place in the other list, dropped somewhere else or nowhere. `sameListCollision` and
  `sameListKeyboardCoordinates` keep a drag among its siblings; onDragEnd's episode → block
  resolution is gone with the case.
- dnd-kit reports the item over itself in the pick-up's own tick; announced, "moved to position 1"
  replaced "picked up … use the arrow keys" before a screen reader read it.

frontend/src/lib/useFocusKeeper.restore.dom.test.tsx

@@ -0,0 +1,64 @@
+// @vitest-environment jsdom
+//
+// Back from a sub-view the focus keeper ALSO sees: the control that held the focus in the sub-view
+// is removed by the swap, the keeper puts the focus on a stand-in in its slot — the first item of
+// the returning list — and `restoreNavPosition` then found the focus "already somewhere" and gave
+// up. Back landed on the first tile instead of the one the view was opened from (measured in Chrome,
+// deep review of the debt burn). Its own file: listNav keeps module-level state.
+import { afterEach, describe, expect, it } from "vitest";
+import { act, cleanup, render } from "@testing-library/react";
+import { useRef, useState } from "react";
+
+import { navItemAttrs, navPosition, restoreNavPosition, useListNav } from "./listNav";
+import { useFocusKeeper } from "./useFocusKeeper";
+
+afterEach(cleanup);
+
+function Grid() {
+  const ref = useRef<HTMLDivElement>(null);
+  useListNav(ref, { enabled: true });
+  return (
+    <div ref={ref}>
+      {["a", "b", "c"].map((t, i) => (
+        <div key={t} {...navItemAttrs(i, t)}>
+          <button data-nav-open="">{t}</button>
+        </div>
+      ))}
+    </div>
+  );
+}
+
+let show: (v: "grid" | "detail") => void = () => {};
+
+function Page() {
+  const ref = useRef<HTMLElement>(null);
+  useFocusKeeper(ref);
+  const [view, setView] = useState<"grid" | "detail">("grid");
+  show = setView;
+  return <main ref={ref}>{view === "grid" ? <Grid /> : <button>Back to library</button>}</main>;
+}
+
+const button = (name: string) =>
+  [...document.querySelectorAll("button")].find((b) => b.textContent === name)!;
+const wait = (ms: number) => act(() => new Promise((r) => setTimeout(r, ms)));
+
+describe("Back from a sub-view whose held control the swap removed", () => {
+  it("returns the focus to the item the view was opened from, not the keeper's stand-in", async () => {
+    render(<Page />);
+    button("c").focus();
+    const pos = navPosition()!;
+    expect(pos.index).toBe(2);
+
+    act(() => show("detail"));
+    button("Back to library").focus(); // a control the keeper holds
+
+    act(() => show("grid")); // popstate's view swap…
+    const cancel = restoreNavPosition(pos); // …and its restore
+    await wait(120);
+    cancel();
+
+    expect(
+      document.activeElement?.closest("[data-nav-index]")?.getAttribute("data-nav-index"),
+    ).toBe("2");
+  });
+});