fix(playlists): an Undo puts a row back after its old neighbour, not at a stale index

A raw index recorded at removal drifted once anything changed before the Undo.
Both lists now read the list fresh and re-insert each row after the row that
preceded it (lib/reinsert). A failed watched toggle in a Plex playlist also
puts the row's state back: its local copy was never re-seeded.

frontend/src/lib/plexWatch.ts

@@ -7,8 +7,10 @@ import { notifyActionError } from "./actionErrors";
 // `resync` (invalidate) — which corrects the optimistic flip to truth either way. Shared by the
 // browse grid and the person page, each of which owns a different cache shape.
 //
-// `revert` is for a cache `resync` must NOT refetch — a random draw (Discover's Surprise rail), which
-// a refetch would reshuffle under the viewer. It is called on a failure only, to put the card back.
+// `revert` is for state `resync` cannot correct, called on a failure only to put the card back: a
+// cache it must NOT refetch — a random draw (Discover's Surprise rail), which a refetch would
+// reshuffle under the viewer — or a component's local copy of the rows (a Plex playlist), which an
+// identical refetch never re-seeds (react-query keeps the same `data`).
 export async function toggleWatchState(
   card: PlexCard,
   t: TFunction,