fix(a11y): keyboard-reorder for panel groups (U-3.1.5)

PanelGroups registered only a PointerSensor, so its focusable, "reorderable" group
grips did nothing from the keyboard. Added KeyboardSensor + sortableKeyboardCoordinates,
matching the other sortable lists (Playlists/PlexPlaylistView already had it).

frontend/src/components/PanelGroups.tsx

@@ -2,12 +2,18 @@ import { type ReactNode } from "react";
 import {
   closestCenter,
   DndContext,
+  KeyboardSensor,
   PointerSensor,
   useSensor,
   useSensors,
   type DragEndEvent,
 } from "@dnd-kit/core";
-import { arrayMove, SortableContext, verticalListSortingStrategy } from "@dnd-kit/sortable";
+import {
+  arrayMove,
+  SortableContext,
+  sortableKeyboardCoordinates,
+  verticalListSortingStrategy,
+} from "@dnd-kit/sortable";
 import type { PanelLayout } from "../lib/panelLayout";
 import PanelGroup from "./PanelGroup";
 
@@ -29,7 +35,12 @@ export default function PanelGroups({
   setLayout: (l: PanelLayout) => void;
   editing: boolean;
 }) {
-  const sensors = useSensors(useSensor(PointerSensor, { activationConstraint: { distance: 4 } }));
+  const sensors = useSensors(
+    useSensor(PointerSensor, { activationConstraint: { distance: 4 } }),
+    // The group grips are focusable and call themselves reorderable; without this only a pointer
+    // could actually move them (keyboard drag matched the other sortable lists).
+    useSensor(KeyboardSensor, { coordinateGetter: sortableKeyboardCoordinates }),
+  );
   const byId = new Map(items.map((it) => [it.id, it]));
 
   const orderedAvailable = layout.order.filter((id) => byId.has(id));