feat(plex): every show card plays the episode it would put on

A show card's picture opened the show page everywhere but Home, whose shelves carried the
resolved `next`. Now every Plex picture plays: a show asks `GET /api/plex/show/{id}/next`
(the in-progress episode, else the first unwatched, else S1·E1 — the rule Home and the show
page's Resume follow) on the click, fresh each time. A started show shows Continue + Restart;
its title, an ⓘ and the menu's "Open show" row open its page; one with no episodes opens there.

The question is asked per show, never carried on every card: three probes a show would cost a
grid page up to 180 queries. The hover preview asks it once it opens and names the episode
("Next: S6·E16 — Title"). Related shows on a show page play the same way.

frontend/src/lib/api/plex.ts

@@ -644,6 +644,10 @@ export const plexApi = {
   },
   plexShow: (id: string, lang?: string): Promise<PlexShowDetail> =>
     req(`/api/plex/show/${encodeURIComponent(id)}${lang === "hu" ? "?lang=hu" : ""}`),
+  // The episode a show's card plays: in progress, else the first unwatched, else the first (null:
+  // no episodes). Asked per show, as a card is hovered or played (the route says why).
+  plexShowNext: (id: string, lang?: string): Promise<{ next: PlexCard | null }> =>
+    req(`/api/plex/show/${encodeURIComponent(id)}/next${lang === "hu" ? "?lang=hu" : ""}`),
   // Mark a whole show / season watched or unwatched (all its episodes) for the current user.
   plexShowState: (rk: string, watched: boolean): Promise<{ changed: number; watched: boolean }> =>
     req(`/api/plex/show/${encodeURIComponent(rk)}/state`, {