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/plexCards.lang.dom.test.tsx

@@ -5,7 +5,9 @@
 // ("Season 1", "Specials") is named from its number instead. i18n here echoes the key AND its values,
 // so an assertion can see which words went into the line.
 import { describe, it, expect, afterEach, vi } from "vitest";
-import { render, screen, cleanup } from "@testing-library/react";
+import { render as rtlRender, screen, cleanup } from "@testing-library/react";
+import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
+import type { ReactElement } from "react";
 
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
@@ -17,6 +19,14 @@ vi.mock("react-i18next", async (importOriginal) => {
 import { PlexPosterCard, SeasonCard } from "./plexCards";
 import type { PlexCard, PlexSeasonDetail } from "../lib/api";
 
+// A show card asks the server which episode it would play once its preview is up: a query client.
+const render = (ui: ReactElement) =>
+  rtlRender(ui, {
+    wrapper: ({ children }) => (
+      <QueryClientProvider client={new QueryClient()}>{children}</QueryClientProvider>
+    ),
+  });
+
 afterEach(cleanup);
 
 const watchedEpisode: PlexCard = {