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/plexShow.aside.dom.test.tsx

@@ -78,6 +78,7 @@ function renderShow(onFilter = vi.fn(), onOpenPerson = vi.fn()) {
         onPlay={() => {}}
         onOpenSeason={() => {}}
         onOpenShow={() => {}}
+        onPlayShow={() => {}}
         onFilter={onFilter}
         onOpenPerson={onOpenPerson}
       />