refactor(plex): scope module state in PlexProvider

Lift the Plex module's shared local state (scope / watch-state / sort / expanded
filters / search term / open-playlist hand-off) out of App into a split-context
PlexProvider with stable useCallback setters. PlexSidebar, PlexBrowse, and the
header search box read it via usePlex()/usePlexActions() instead of props drilled
through App; PlexBrowse is now prop-free. App loses six useStates + the filters
JSON memo/serialise helpers. (Platform S6b.2)

frontend/src/components/PlexBrowse.tsx

@@ -34,6 +34,7 @@ import { DetailCustomizeMenu, Filterable, useArtBackdrop, useDetailPrefs } from
 import PlexPlaylistAdd, { type PlexAddTarget } from "./PlexPlaylistAdd";
 import PlexCollectionEditor from "./PlexCollectionEditor";
 import { PageToolbar } from "./PageShell";
+import { usePlex, usePlexActions } from "./PlexProvider";
 
 // Lazy: the rich player (pulls in hls.js) loads only when something is first played.
 const PlexPlayer = lazy(() => import("./PlexPlayer"));
@@ -54,34 +55,16 @@ type Sub =
 // (useHistorySubview) so browser Back returns to the grid. Playback lands in P2 — a card announces
 // for now. Rendered by App for page==="plex" (its own nav module).
 
-type Props = {
-  q: string;
-  scope: string; // movie | show | both (unified cross-library scope)
-  setScope: (v: string) => void;
-  show: string;
-  sort: string;
-  filters: PlexFilters;
-  setFilters: (f: PlexFilters) => void;
-  // The sidebar opens a playlist by setting this; PlexBrowse turns it into a history subview + clears it.
-  openPlaylist?: number | null;
-  onPlaylistOpened?: () => void;
-};
-
 const PAGE = 40;
 
-export default function PlexBrowse({
-  q,
-  scope,
-  setScope,
-  show,
-  sort,
-  filters,
-  setFilters,
-  openPlaylist,
-  onPlaylistOpened,
-}: Props) {
+export default function PlexBrowse() {
   const { t } = useTranslation();
   const qc = useQueryClient();
+  // Scope / watch-state / sort / filters live in PlexProvider (edited mainly from PlexSidebar); the
+  // search term (q) comes from the shared header box; the rail hands off a playlist to open via the
+  // shared `playlistOpen` slot, which we consume into a history subview then clear.
+  const { q, scope, show, sort, filters, playlistOpen: openPlaylist } = usePlex();
+  const { setScope, setFilters, setPlaylistOpen } = usePlexActions();
   const dq = useDebounced(q.trim(), 350);
   const sub = useHistorySubview<Sub>({ kind: "grid" });
 
@@ -90,7 +73,7 @@ export default function PlexBrowse({
   useEffect(() => {
     if (openPlaylist != null) {
       sub.open({ kind: "playlist", id: openPlaylist });
-      onPlaylistOpened?.();
+      setPlaylistOpen(null);
     }
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [openPlaylist]);