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/keymap.ts

@@ -31,13 +31,19 @@ export type KeyScope =
   | "page"
   | "search"
   | "list"
+  | "listDelete"
   | "undo"
   | "contextMenu"
   | "ytPlayer"
   | "plexPlayer"
   | "plexBrowse";
 
-const PAGE_LEVEL: ReadonlySet<KeyScope> = new Set<KeyScope>(["page", "search", "list"]);
+const PAGE_LEVEL: ReadonlySet<KeyScope> = new Set<KeyScope>([
+  "page",
+  "search",
+  "list",
+  "listDelete",
+]);
 /** True for a scope that acts on the page beneath the overlays (see `KeyScope`). */
 export const isPageLevelScope = (scope: KeyScope): boolean => PAGE_LEVEL.has(scope);
 
@@ -132,6 +138,9 @@ export const SHORTCUTS = [
   { id: "up", scope: "list", keys: ["ArrowUp"], labelKey: "keys.gridMove" },
   { id: "down", scope: "list", keys: ["ArrowDown"], labelKey: "keys.gridMove" },
   { id: "open", scope: "list", keys: ["Enter"], labelKey: "keys.open" },
+  // Its own scope beside `list`: only a list that can take an item out (a playlist's rows) registers
+  // it, so the sheet does not promise Delete on the feed or a Plex grid, where nothing answers it.
+  { id: "remove", scope: "listDelete", keys: ["Delete"], labelKey: "keys.listRemove" },
   // --- the row context menu (wherever useContextMenu is wired) ---------------------------------
   // Its OWN scope, not `global`: only the surfaces that actually call `useContextMenu` register it,
   // so the sheet stops advertising "Open the context menu" on the feed, Plex, Downloads and Settings
@@ -412,7 +421,9 @@ export function useActiveScopes(): KeyScope[] {
   // registers `page` in its own effect, which React runs after every child's — so by mount order
   // the module jumps would head the sheet, above the list keys of the page you are looking at.
   const surfaces = live.filter((s) => !isPageLevelScope(s));
-  const pageLevel = (["list", "search", "page"] as const).filter((s) => live.includes(s));
+  const pageLevel = (["list", "listDelete", "search", "page"] as const).filter((s) =>
+    live.includes(s),
+  );
   return [...surfaces, ...pageLevel, "global"];
 }