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/components/ui/MediaPreview.tsx

@@ -18,12 +18,15 @@ export function MediaPreview({
   preview,
   title,
   facts,
+  note,
   summary,
 }: {
   preview: ReturnType<typeof useMediaPreview>;
   title: string;
   /** One line under the title: year, length, score. */
   facts?: ReactNode;
+  /** A line of its own under the facts (a show's "Next: S6·E16 — Title"). */
+  note?: ReactNode;
   summary?: string | null;
 }) {
   return (
@@ -36,6 +39,7 @@ export function MediaPreview({
     >
       <p className="text-sm font-semibold leading-snug text-fg">{title}</p>
       {facts && <div className="mt-0.5 text-xs text-muted">{facts}</div>}
+      {note && <p className="mt-1 text-xs font-medium text-fg">{note}</p>}
       {summary && (
         <p className="mt-2 line-clamp-8 whitespace-pre-line text-xs leading-relaxed text-fg">
           {summary}