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/tableHeader.tsx

@@ -219,7 +219,7 @@ export function ColumnHead<T>({
             <th
               key={col.key}
               style={col.width ? { width: col.width } : undefined}
-              className={`relative font-medium px-2 py-2 whitespace-nowrap bg-bg ${alignClass(col.align)}`}
+              className={`relative font-medium px-2 py-2 whitespace-nowrap bg-bg ${alignClass(col.align)}${col.phoneHidden ? " hidden sm:table-cell" : ""}`}
               aria-sort={
                 col.sortable
                   ? isSorted