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/lib/plexDetailUi.tsx

@@ -125,7 +125,7 @@ export function useCrossfadeBackdrop(
     const pool = key ? key.split("|") : [];
     if (!enabled || pool.length === 0) {
       clear();
-      return;
+      return clear; // and clear again on unmount / dep-change (idempotent)
     }
     paint(pool[0]!);
 
@@ -133,7 +133,9 @@ export function useCrossfadeBackdrop(
       typeof window !== "undefined" &&
       typeof window.matchMedia === "function" &&
       window.matchMedia("(prefers-reduced-motion: reduce)").matches;
-    if (!animate || reduce || pool.length < 2) return; // static: first image only
+    // Static case (one image / crossfade off / reduced motion): still MUST clear on unmount — otherwise
+    // #root keeps the painted backdrop after leaving the page (e.g. switching to another module).
+    if (!animate || reduce || pool.length < 2) return clear;
 
     const layer = document.createElement("div");
     Object.assign(layer.style, {