test(e4): lock accessible names on the Plex card buttons

Closes the re-review's test-coverage finding: a plexCards.dom.test asserting the
icon-only card controls (watched-toggle, Info, add-season/episode) each expose
their aria-label as an accessible name via getByRole({name}). A future edit that
drops a label now fails the gate instead of silently reopening the unnamed-button
gap the a11y sweep closed.

4 tests; full FE gate green (tsc/eslint/prettier/knip, 321 vitest).

frontend/src/components/plexCards.dom.test.tsx

@@ -0,0 +1,82 @@
+// @vitest-environment jsdom
+//
+// Plex card a11y: locks the accessible names on the icon-only card controls (the E4 a11y sweep). These
+// buttons are icon-only (WatchedIcon / Info / ListPlus, no text), so their name comes solely from the
+// 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 } from "@testing-library/react";
+
+vi.mock("react-i18next", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("react-i18next")>();
+  return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };
+});
+
+import { PlexPosterCard, SeasonCard, EpisodeCard } from "./plexCards";
+import type { PlexCard, PlexSeasonDetail } from "../lib/api";
+
+const movie = (status?: string): PlexCard => ({
+  id: "1",
+  type: "movie",
+  title: "M",
+  thumb: "",
+  status,
+});
+const episode = (status?: string): PlexCard => ({
+  id: "e1",
+  type: "episode",
+  title: "E1",
+  thumb: "",
+  status,
+});
+const season = (status = "new"): PlexSeasonDetail => ({
+  id: "s1",
+  season_number: 1,
+  title: "S1",
+  thumb: "",
+  episode_count: 3,
+  status,
+  episodes: [],
+});
+
+afterEach(cleanup);
+
+describe("PlexPosterCard a11y", () => {
+  it("names the watched-toggle (mark-watched when unwatched) and the info button", () => {
+    render(
+      <PlexPosterCard card={movie()} onOpen={vi.fn()} onInfo={vi.fn()} onToggleWatched={vi.fn()} />,
+    );
+    expect(screen.getByRole("button", { name: "plex.markWatched" })).toBeTruthy();
+    expect(screen.getByRole("button", { name: "plex.info.title" })).toBeTruthy();
+  });
+
+  it("names the watched-toggle as mark-unwatched when the movie is watched", () => {
+    render(
+      <PlexPosterCard
+        card={movie("watched")}
+        onOpen={vi.fn()}
+        onInfo={vi.fn()}
+        onToggleWatched={vi.fn()}
+      />,
+    );
+    expect(screen.getByRole("button", { name: "plex.markUnwatched" })).toBeTruthy();
+  });
+});
+
+describe("SeasonCard a11y", () => {
+  it("names the watched-toggle and the add-season button", () => {
+    render(<SeasonCard se={season()} onOpen={vi.fn()} onToggleWatched={vi.fn()} onAdd={vi.fn()} />);
+    expect(screen.getByRole("button", { name: "plex.series.markSeasonWatched" })).toBeTruthy();
+    expect(screen.getByRole("button", { name: "plex.playlist.addSeason" })).toBeTruthy();
+  });
+});
+
+describe("EpisodeCard a11y", () => {
+  it("names the watched-toggle and the add-episode button", () => {
+    render(
+      <EpisodeCard ep={episode()} onPlay={vi.fn()} onToggleWatched={vi.fn()} onAdd={vi.fn()} />,
+    );
+    expect(screen.getByRole("button", { name: "plex.markWatched" })).toBeTruthy();
+    expect(screen.getByRole("button", { name: "plex.playlist.addEpisode" })).toBeTruthy();
+  });
+});