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/PlexPlaylistView.tsx

@@ -6,17 +6,11 @@ import {
   DndContext,
   KeyboardSensor,
   PointerSensor,
-  closestCenter,
   useSensor,
   useSensors,
   type DragEndEvent,
 } from "@dnd-kit/core";
-import {
-  SortableContext,
-  arrayMove,
-  sortableKeyboardCoordinates,
-  verticalListSortingStrategy,
-} from "@dnd-kit/sortable";
+import { SortableContext, arrayMove, verticalListSortingStrategy } from "@dnd-kit/sortable";
 import { CSS } from "@dnd-kit/utilities";
 import {
   ArrowLeft,
@@ -40,7 +34,12 @@ import { LoadingState, StateMessage } from "./ui/QueryState";
 import ViewSwitcher, { type ViewOption } from "./ViewSwitcher";
 import { FocusSafeButton } from "./ui/FocusSafeButton";
 import { usePlexPlaylistEdits } from "./usePlexPlaylistEdits";
-import { useSortableA11y, useSortableItem } from "./ui/sortableA11y";
+import {
+  sameListCollision,
+  sameListKeyboardCoordinates,
+  useSortableA11y,
+  useSortableItem,
+} from "./ui/sortableA11y";
 
 // A playlist's items, grouped for readability: movies stay standalone, while a run of episodes from the
 // same show collapses into a season/show group so a whole series doesn't sprawl into an endless flat
@@ -455,7 +454,7 @@ export default function PlexPlaylistView({
 
   const sensors = useSensors(
     useSensor(PointerSensor, { activationConstraint: { distance: 4 } }),
-    useSensor(KeyboardSensor, { coordinateGetter: sortableKeyboardCoordinates }),
+    useSensor(KeyboardSensor, { coordinateGetter: sameListKeyboardCoordinates }),
   );
 
   // A whole block (a movie, a show) sorts among the blocks; an episode only within its own show.
@@ -481,20 +480,16 @@ export default function PlexPlaylistView({
     const aId = String(active.id);
     const oId = String(over.id);
     const blockIds = new Set(blocks.map((b) => b.id));
+    // `over` is always a sibling of `active` (sameListCollision): a block over a block, an episode
+    // over an episode of the same show. The checks below only refuse a stale id.
     if (blockIds.has(aId)) {
-      // Moving a whole block (movie or show). Resolve the drop target to a block.
-      let oBlockId = oId;
-      if (!blockIds.has(oId)) {
-        const b = blocks.find((bl) => bl.kind === "show" && bl.items.some((it) => it.id === oId));
-        if (!b) return;
-        oBlockId = b.id;
-      }
+      // Moving a whole block (movie or show) among the blocks.
       const from = blocks.findIndex((b) => b.id === aId);
-      const to = blocks.findIndex((b) => b.id === oBlockId);
+      const to = blocks.findIndex((b) => b.id === oId);
       if (from < 0 || to < 0) return;
       commit(flatten(arrayMove(blocks, from, to)));
     } else {
-      // Moving an episode — only within its own show block.
+      // Moving an episode — within its own show block.
       const b = blocks.find((bl) => bl.kind === "show" && bl.items.some((it) => it.id === aId));
       if (!b || b.kind !== "show" || !b.items.some((it) => it.id === oId)) return;
       const from = b.items.findIndex((it) => it.id === aId);
@@ -600,7 +595,7 @@ export default function PlexPlaylistView({
         ) : (
           <DndContext
             sensors={sensors}
-            collisionDetection={closestCenter}
+            collisionDetection={sameListCollision}
             onDragEnd={onDragEnd}
             accessibility={dndA11y}
           >