feat(design): a playlist's one action list (C1) — the row's "…", Delete + undo, the header's "…" (E28 S3e2)

- Playlist rows: the X column became the row's "…" (play, pin, copy title/link, move, order,
  remove). Remove runs at once, with no confirm; its toast's Undo puts the row back at its index.
  The Delete key removes a keyboard-walked row, and focus moves on to the next.
- POST /api/playlists/{id}/items takes an optional `position`, so an undo is one request that
  renumbers the list around the row.
- A new `listDelete` key scope. Only a list with `onDelete` registers it, so the help sheet does not
  offer Delete where nothing handles it.
- A playlist has one list, `usePlaylistActions.items`: rename, pin, YouTube open/push/revert,
  delete. The rail row's "…" and the page header's "…" both open it. Push and revert moved into
  the hook; the header keeps Play all.
- Phone-width playlist table: "#" and Channel are hidden below sm (`Column.phoneHidden`). Their
  fixed widths had left the title no width.

frontend/src/components/ui/NavGrid.tsx

@@ -10,14 +10,17 @@ import { useListNav } from "../../lib/listNav";
 export default function NavGrid({
   as: Tag = "div",
   className,
+  onDelete,
   children,
 }: {
   as?: "div" | "tbody";
   className?: string;
+  /** The Delete key takes the focused item out (lib/listNav): its index among the items. */
+  onDelete?: (index: number) => void;
   children: ReactNode;
 }) {
   const ref = useRef<HTMLDivElement & HTMLTableSectionElement>(null);
-  useListNav(ref, { enabled: true });
+  useListNav(ref, { enabled: true, onDelete });
   return (
     <Tag ref={ref} className={className}>
       {children}