feat(ui): E31 S4c — the channel header folds, the Plex pages fill their hero, the player gets a side panel

- Channel page: the banner and identity block fold away once the grid scrolls
  (ui/scrollFold: folds past 32px only if the page can stay scrolled, unfolds
  at the very top) and the name rides in the tab row. One Back button in both
  states keeps its focus; folded at a tablet width the actions wrap instead of
  pushing the page sideways.
- Plex movie and show pages: the TMDB facts over the key crew are the hero's
  second column (plexDetailAside), the keywords sit under the text; the show
  page shows them for the first time, with the same three toggles. A
  collection's hero is sized to its text.
- YouTube player, 1280px and wider: the title, channel, actions, description
  and queue sit in a side panel beside the video, which takes the window's
  height (1438x809 at 1920x911, was ~1008 wide). The wheel scrolls that panel
  while it can and turns the volume elsewhere.
- DESIGN.md decision log: the keywords' place under the text.

frontend/src/lib/plexDetail.ts

@@ -1,7 +1,7 @@
 import { useLayoutEffect } from "react";
 import { usePref, useSavePref } from "./usePref";
 import type { TFunction } from "i18next";
-import type { PlexCard } from "./api";
+import type { PlexCard, PlexTmdb } from "./api";
 
 // The non-component half of the shared Plex "detail page" UI (see plexDetailUi): the per-user display
 // prefs, the TMDB backdrop URL, the crossfading art backdrop hook and the episode label. Split out so
@@ -31,6 +31,56 @@ export function useDetailPrefs() {
   };
 }
 
+// What a detail page's facts are read from: a movie's item detail and a show's detail both carry these.
+export interface DetailFacts {
+  year?: number | null;
+  duration_seconds?: number | null;
+  content_rating?: string | null;
+  studio?: string | null;
+  tmdb?: PlexTmdb | null;
+}
+
+export type AsideSection = "facts" | "keywords" | "crew";
+
+// The detail pages' second column (plexDetailAside, E31): which of its three sections — the TMDB facts,
+// keywords and key crew — have something to show, which the reader keeps on (three per-user prefs,
+// default ON, the same on the movie and the show page), and the toggles' flips.
+export function useDetailAside(f: DetailFacts) {
+  const [facts, setFacts] = usePref<boolean>("plexInfoFacts", true);
+  const [keywords, setKeywords] = usePref<boolean>("plexInfoKeywords", true);
+  const [crew, setCrew] = usePref<boolean>("plexInfoCrew", true);
+  const has: Record<AsideSection, boolean> = {
+    facts:
+      f.year != null ||
+      !!f.duration_seconds ||
+      !!f.content_rating ||
+      !!f.tmdb?.budget ||
+      !!f.tmdb?.revenue ||
+      !!f.tmdb?.studios?.length ||
+      !!f.studio ||
+      !!f.tmdb?.production_countries?.length,
+    keywords: (f.tmdb?.keywords?.length ?? 0) > 0,
+    crew: (f.tmdb?.crew?.length ?? 0) > 0,
+  };
+  const on: Record<AsideSection, boolean> = { facts, keywords, crew };
+  const shown: Record<AsideSection, boolean> = {
+    facts: facts && has.facts,
+    keywords: keywords && has.keywords,
+    crew: crew && has.crew,
+  };
+  const set = { facts: setFacts, keywords: setKeywords, crew: setCrew };
+  return {
+    has,
+    on,
+    shown,
+    // Whether the hero's second column has anything (the keywords sit under the text).
+    column: shown.facts || shown.crew,
+    toggle: (k: AsideSection) => set[k](!on[k]),
+  };
+}
+
+export type DetailAsideState = ReturnType<typeof useDetailAside>;
+
 // A stored TMDB backdrop is a bare file path ("/xyz.jpg"); the CSP allows the TMDB image CDN (only
 // frame-ancestors is set), so we render it straight from there — a rich multi-image pool for the
 // crossfade even on a lone movie with no collection.