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/PlexMenu.tsx

@@ -28,7 +28,7 @@ export function PlexMenu({
       <ContextMenu
         menu={menu}
         ariaLabel={t("plex.menu")}
-        items={c ? [...actions.items(c, { pin }), ...(extra?.(c) ?? [])] : []}
+        items={c ? [...actions.items(c, { pin, anchor: menu.anchor }), ...(extra?.(c) ?? [])] : []}
       />
       {actions.panel}
     </>