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/components/Home.tsx

@@ -199,7 +199,7 @@ function useHomeCardMenu({
               .map((it, i) => (i === 0 ? { ...it, separatorBefore: true } : it))
           : target.plex
             ? plexActions
-                .items(target.plex, { play: false, pin: false })
+                .items(target.plex, { play: false, pin: false, anchor: menu.anchor })
                 .map((it, i) => (i === 0 ? { ...it, separatorBefore: true } : it))
             : target.kind === "channel" || target.kind === "playlist"
               ? [{ ...openPage.item(target), separatorBefore: true }]