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/lib/listNav.scope.dom.test.tsx

@@ -16,9 +16,9 @@ function Scopes() {
   return <output data-testid="scopes">{useActiveScopes().join(" ")}</output>;
 }
 
-function Queue({ count }: { count: number }) {
+function Queue({ count, onDelete }: { count: number; onDelete?: (i: number) => void }) {
   const ref = useRef<HTMLDivElement>(null);
-  useListNav(ref, { enabled: true });
+  useListNav(ref, { enabled: true, onDelete });
   return (
     <div ref={ref}>
       {count === 0 ? (
@@ -34,10 +34,10 @@ function Queue({ count }: { count: number }) {
   );
 }
 
-const page = (count: number) => (
+const page = (count: number, onDelete?: (i: number) => void) => (
   <>
     <Scopes />
-    <Queue count={count} />
+    <Queue count={count} onDelete={onDelete} />
   </>
 );
 const scopes = () => screen.getByTestId("scopes").textContent!.split(" ");
@@ -51,4 +51,16 @@ describe("useListNav · the list key scope", () => {
     rerender(page(0));
     await waitFor(() => expect(scopes()).not.toContain("list"));
   });
+
+  // E28 S3e2: Delete is on the sheet only where a list can take an item out — a playlist's rows, not
+  // the feed or a Plex grid, where nothing answers it.
+  it("puts Delete on the sheet only for a list that removes items", async () => {
+    const { rerender } = render(page(2));
+    await waitFor(() => expect(scopes()).toContain("list"));
+    expect(scopes()).not.toContain("listDelete");
+    rerender(page(2, () => {}));
+    await waitFor(() => expect(scopes()).toContain("listDelete"));
+    rerender(page(0, () => {}));
+    await waitFor(() => expect(scopes()).not.toContain("listDelete"));
+  });
 });