E5 S1: shared Carousel primitive + Plex card-art serializer

Wave 5a building blocks under the Seerr-inspired Plex enrichment (E5).

Carousel (frontend/src/components/Carousel.tsx): the one horizontal-rail
component, replacing the hand-rolled `flex overflow-x-auto` strips and the
home of the future Plex-Discover rows. Edge-fade mask, hover/focus scroll
buttons that appear only on the clipped side (useHScroll — the X-axis twin
of useScrollFade, same watch-the-children discipline), native scrollbar
hidden behind .no-scrollbar, reduced-motion-aware paging. First consumers:
CastStrip + RelatedStrip now scroll inside it (dedup + free affordances).
New i18n: common.seeAll / scrollLeft / scrollRight (en+hu).

Card-art serializer (backend/app/plex/cards.py): movie_card + show_card now
emit `art` (the wide backdrop, /api/plex/image/{rk}?variant=art) so rails and
the crossfading hero get a landscape image with no per-item fetch — gated on
art_key so an art-less item stays null and falls back to the poster. FE type
PlexCard.art?.

Tests: Carousel.dom.test (children/header/see-all, clipped-edge buttons +
paging), test_plex_cards (art present/absent by art_key). Added a
ResizeObserver stub to PlexPersonView.dom.test (it renders CastStrip, now a
Carousel consumer).

Gate: tsc x3 / eslint / prettier / knip / vitest 324 green; ruff green;
backend test lane (docker-compose.test.yml) green on the plex card tests.

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

@@ -5,10 +5,23 @@
 // filter gesture only when no onOpenPerson is wired) and PlexPersonView's render (headshot/name hero,
 // the Movies/Shows sections, and the "filter library by this person" action). i18n is key-passthrough
 // and the api is mocked so the query resolves synchronously in the test.
-import { describe, it, expect, afterEach, vi } from "vitest";
+import { describe, it, expect, afterEach, beforeAll, vi } from "vitest";
 import { render, screen, fireEvent, cleanup } from "@testing-library/react";
 import { QueryClient, QueryClientProvider } from "@tanstack/react-query";
 
+beforeAll(() => {
+  // CastStrip now scrolls inside a Carousel, whose useHScroll wires a ResizeObserver; jsdom ships none.
+  class Stub {
+    observe() {}
+    unobserve() {}
+    disconnect() {}
+    takeRecords() {
+      return [];
+    }
+  }
+  globalThis.ResizeObserver ??= Stub as unknown as typeof ResizeObserver;
+});
+
 vi.mock("react-i18next", async (importOriginal) => {
   const actual = await importOriginal<typeof import("react-i18next")>();
   return { ...actual, useTranslation: () => ({ t: (key: string) => key }) };