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.dom.test.tsx

@@ -46,6 +46,29 @@ describe("useFocusKeeper", () => {
     expect(document.activeElement).toBe(btn("after the list"));
   });
 
+  // A page swapped for its loading fallback leaves nothing focusable in the slot. The walk up used to
+  // run past the container and hand the focus to the first control of the whole document.
+  it("never puts the focus outside its own container", async () => {
+    function Swapped() {
+      const ref = useRef<HTMLDivElement>(null);
+      useFocusKeeper(ref);
+      const [loaded, setLoaded] = useState(true);
+      return (
+        <>
+          <button>rail</button>
+          <div ref={ref}>
+            {loaded ? <button onClick={() => setLoaded(false)}>go</button> : <p>Loading…</p>}
+          </div>
+        </>
+      );
+    }
+    render(<Swapped />);
+    btn("go").focus();
+    act(() => btn("go").click());
+    await flush();
+    expect(document.activeElement).toBe(document.body);
+  });
+
   it("leaves a keyboard list's rows to listNav", async () => {
     render(<Rows nav />);
     btn("approve b").focus();