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.

backend/app/plex/cards.py

@@ -6,6 +6,14 @@ router. Pure functions — no DB, no request state."""
 from app.models import PlexItem, PlexShow, PlexState
 
 
+# A card's wide backdrop, for a rail's landscape tile or a crossfading hero — but ONLY when the item
+# actually has art. `thumb` is always emitted (its endpoint placeholders a missing poster), but an
+# `art` URL for an item with no `art_key` would resolve to a broken/blank image on every card that
+# tried to use it, so gate it on the stored key and let the client fall back to the poster.
+def _art_url(rating_key: str, art_key: str | None) -> str | None:
+    return f"/api/plex/image/{rating_key}?variant=art" if art_key else None
+
+
 def movie_card(it: PlexItem, st: PlexState | None) -> dict:
     return {
         "id": it.rating_key,
@@ -14,6 +22,7 @@ def movie_card(it: PlexItem, st: PlexState | None) -> dict:
         "year": it.year,
         "duration_seconds": it.duration_s,
         "thumb": f"/api/plex/image/{it.rating_key}",
+        "art": _art_url(it.rating_key, it.art_key),
         "playable": it.playable,
         "status": st.status if st else "new",
         "position_seconds": st.position_seconds if st else 0,
@@ -27,6 +36,7 @@ def show_card(sh: PlexShow, status: str = "new") -> dict:
         "title": sh.title,
         "year": sh.year,
         "thumb": f"/api/plex/image/{sh.rating_key}",
+        "art": _art_url(sh.rating_key, sh.art_key),
         "season_count": sh.child_count,
         # Aggregate watch-state across the show's episodes (new | in_progress | watched) → grid badge.
         "status": status,