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/components/ui/DataTable.tsx

@@ -138,8 +138,11 @@ export default function DataTable<T>({
   const wide = useMediaQuery("(min-width: 48rem)");
   const bodyRef = useRef<HTMLTableSectionElement>(null);
   const cardsRef = useRef<HTMLDivElement>(null);
-  useListNav(bodyRef, { enabled: keyboardNav });
-  useListNav(cardsRef, { enabled: keyboardNav });
+  // Enabled with the layout that is mounted: `useListNav` reads its ref when `enabled` changes, so a
+  // flag that stayed on across a breakpoint left the newly mounted layout unregistered (no j/k) and
+  // the unmounted one registered.
+  useListNav(bodyRef, { enabled: keyboardNav && wide });
+  useListNav(cardsRef, { enabled: keyboardNav && !wide });
 
   // Commit a whole new filter map through the owner (controlled) or internal state. A ref holds the
   // latest resolved map so the externalFilter/reset effects can derive from it without taking