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.dom.test.tsx

@@ -5,7 +5,9 @@
 // aria-label — a dropped label silently reintroduces an unnamed button for screen-reader users. i18n is
 // key-passthrough, so the asserted names are the i18n keys.
 import { describe, it, expect, afterEach, vi } from "vitest";
-import { render, screen, cleanup, fireEvent, act } from "@testing-library/react";
+import { render as rtlRender, screen, cleanup, fireEvent, act } 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")>();
@@ -21,7 +23,15 @@ vi.mock("react-i18next", async (importOriginal) => {
 
 import { PlexPosterCard, SeasonCard, EpisodeCard } from "./plexCards";
 import { iconButtonClass } from "./ui/buttonStyles";
-import type { PlexCard, PlexSeasonDetail } from "../lib/api";
+import { api, type PlexCard, type 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>
+    ),
+  });
 
 const movie = (status?: string): PlexCard => ({
   id: "1",
@@ -169,14 +179,57 @@ describe("PlexPosterCard with a next episode", () => {
     expect(onInfo).toHaveBeenCalled();
   });
 
-  it("stays a plain show card without one: no Play, no toggles, no ⓘ", () => {
+  // Every show card plays (the owner, 2026-10-09): without its episode in hand the surface's onOpen
+  // asks which one. The toggles act on an episode, so they wait for one; the ⓘ is the show's page.
+  it("plays without one too, opens its page from the ⓘ, and offers no toggles", () => {
+    const onOpen = vi.fn();
+    const onPage = vi.fn();
     const { container } = render(
-      <PlexPosterCard card={show} onOpen={vi.fn()} onToggleWatched={vi.fn()} onInfo={vi.fn()} />,
+      <PlexPosterCard card={show} onOpen={onOpen} onPage={onPage} onToggleWatched={vi.fn()} />,
     );
-    expect(playGlyph(container)).toBeNull();
-    expect(screen.queryByRole("button", { name: "plex.openShow" })).toBeNull();
+    expect(playGlyph(container)).not.toBeNull();
+    fireEvent.click(screen.getByRole("button", { name: "Show" }));
+    expect(onOpen).toHaveBeenCalledTimes(1);
+    fireEvent.click(screen.getByRole("button", { name: "plex.openShow" }));
+    expect(onPage).toHaveBeenCalledWith(show);
     expect(screen.queryByRole("button", { name: "plex.markWatched" })).toBeNull();
   });
+
+  it("continues a started show and starts its episode over, as a started movie does", () => {
+    const onOpen = vi.fn();
+    const onRestart = vi.fn();
+    render(
+      <PlexPosterCard
+        card={{ ...show, status: "in_progress" }}
+        onOpen={onOpen}
+        onRestart={onRestart}
+      />,
+    );
+    fireEvent.click(screen.getByRole("button", { name: "card.restart" }));
+    expect(onRestart).toHaveBeenCalledTimes(1);
+    fireEvent.click(screen.getByRole("button", { name: "card.continue" }));
+    expect(onOpen).toHaveBeenCalledTimes(1);
+  });
+
+  it("names the episode it would play once its preview is up, and not asked before", async () => {
+    vi.useFakeTimers();
+    const ask = vi
+      .spyOn(api, "plexShowNext")
+      .mockResolvedValue({ next: { ...next, title: "Pilot" } });
+    render(<PlexPosterCard card={show} onOpen={vi.fn()} />);
+    fireEvent.mouseEnter(screen.getByRole("button", { name: "Show" }).closest(".cv-poster")!);
+    act(() => {
+      vi.advanceTimersByTime(600);
+    });
+    expect(ask).not.toHaveBeenCalled(); // a sweep across a grid asks nothing
+    await act(async () => {
+      vi.advanceTimersByTime(400);
+    });
+    vi.useRealTimers();
+    expect(ask).toHaveBeenCalledWith("sh", "en");
+    expect(await screen.findByText("plex.nextUp:S6·E16 — Pilot")).toBeTruthy();
+    ask.mockRestore();
+  });
 });
 
 // C5: a started item's picture offers Continue and Restart (the video card's pair) instead of a Play;