fix(e5): Plex detail/UI polish from review (backdrop reset, layout, combobox, labels)

User-reported issues after the E5 build:

- useCrossfadeBackdrop: the static branch (one image / crossfade off /
  reduced motion) painted #root but returned NO cleanup, so the backdrop
  persisted after leaving the page (e.g. switching modules). Every painted
  branch now returns clear() so unmount always resets the default backdrop.
- scheduler: give the tmdb_enrich job a proper label + description (was the
  raw "tmdb_enrich" id) in EN/HU.
- CastStrip: cap crew names to ONE line (truncate) so the row of circular
  photos sits on one baseline instead of going wavy on 2-line names.
- PlexInfo: lay Details / Keywords / Crew out as side-by-side boxes
  (items-start grid) instead of full-width stacked panels with dead space;
  narrower Crew grid for the boxed width; each panel independently hideable
  from the customize menu (new Details/Keywords/Crew toggles). Tighter
  vertical spacing.
- MultiSelectChips: reshape the trigger from a "+Add" button into an
  Overseerr-style combi input — removable chips inside a typeable field that
  filters the checklist, opening on focus (backspace removes the last chip).
- Discover / person rails: reduce the vertical gap between rails.

frontend/src/components/PlexDiscover.tsx

@@ -56,7 +56,7 @@ function CardRail({
       title
     );
   return (
-    <section className="mb-8">
+    <section className="mb-5">
       <Carousel title={heading} ariaLabel={ariaLabel} onSeeAll={onSeeAll} seeAllLabel={seeAllLabel}>
         {cards.map((c) => (
           <div key={c.id} className="w-[150px] shrink-0">
@@ -198,7 +198,7 @@ export default function PlexDiscover({
       )}
 
       {d.genres.length > 0 && (
-        <section className="mb-8">
+        <section className="mb-5">
           <h2 className="mb-3 text-sm font-semibold">{t("plex.discover.byGenre")}</h2>
           <div className="grid grid-cols-2 gap-3 sm:grid-cols-3 md:grid-cols-4 lg:grid-cols-6">
             {d.genres.map((g) => {