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/useMediaPreview.ts

@@ -19,6 +19,8 @@ export function useMediaPreview(enabled = true) {
   });
   return {
     popover: pop,
+    /** The preview is up: the pointer has rested on the card (what a card may fetch for it waits on). */
+    open: open && enabled,
     /** The card root's props: the anchor, the hover wiring, and a close on any press — merged with
      *  the root's own (its menu's right-click and long-press). */
     rootProps: (own: HTMLAttributes<HTMLDivElement> & Record<string, unknown> = {}) => ({