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/api/playlists.ts

@@ -68,10 +68,12 @@ export const playlistsApi = {
     req(`/api/playlists/${id}`, { method: "PATCH", body: JSON.stringify({ name }) }),
   deletePlaylist: (id: number, onYoutube = false) =>
     req(`/api/playlists/${id}${onYoutube ? "?on_youtube=true" : ""}`, { method: "DELETE" }),
-  addToPlaylist: (id: number, videoId: string) =>
+  // `position` (an index in the stored order) puts it back where a removal took it from — the
+  // list page's Undo; omitted, it is appended.
+  addToPlaylist: (id: number, videoId: string, position?: number) =>
     req(`/api/playlists/${id}/items`, {
       method: "POST",
-      body: JSON.stringify({ video_id: videoId }),
+      body: JSON.stringify({ video_id: videoId, ...(position != null && { position }) }),
     }),
   removeFromPlaylist: (id: number, videoId: string) =>
     req(`/api/playlists/${id}/items/${videoId}`, { method: "DELETE" }),