feat(plex): add a movie or show to a collection from its card menu

"Add to collection…" opens a panel beside the card: the title's library's
manual collections tick it in or out, and a name + Enter creates a new one with
the title in it. It used to take the Collections page, a new collection, its
name and a title search, then another search per title. A lean endpoint lists
the collections with whether each holds the title. Admin only, as on the
title's page.

Also: a collection seeded with a show is made as a show collection (Plex type
2). Measured: as a movie one it came out empty and refused the show after.

frontend/src/lib/api/plex.ts

@@ -508,6 +508,12 @@ export const plexApi = {
     const qs = u.toString();
     return req(`/api/plex/collections${qs ? `?${qs}` : ""}`);
   },
+  // A movie or show card's "Add to collection": its library's manual collections, each saying whether
+  // the title is in it already.
+  plexCollectionsFor: (
+    itemRk: string,
+  ): Promise<{ collections: (PlexCollection & { contains: boolean })[] }> =>
+    req(`/api/plex/collections/for/${encodeURIComponent(itemRk)}`),
   // One collection's hero + its members in the requested order (the Collection info page). The sort
   // vocabulary is the browse grid's, so the same Sort control means the same thing on both.
   plexCollection: (rk: string, sort?: SortState, lang?: string): Promise<PlexCollectionDetail> => {