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.
