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/PlexSidebar.tsx

@@ -8,6 +8,7 @@ import { useDebounced } from "../lib/useDebounced";
 import { useScrollFade } from "../lib/useScrollFade";
 import SidePanel from "./SidePanel";
 import PanelGroups, { type PanelItem } from "./PanelGroups";
+import { usePlex, usePlexActions } from "./PlexProvider";
 
 // The Plex module's left filter column — now the shared floating SidePanel with its filter groups
 // as reorderable "islands" (same system as the feed rail). Movie libraries get the full metadata
@@ -15,15 +16,6 @@ import PanelGroups, { type PanelItem } from "./PanelGroups";
 // the backend so the panel only offers what the library actually contains.
 
 type Props = {
-  scope: string; // movie | show | both (unified cross-library scope)
-  setScope: (v: string) => void;
-  show: string;
-  setShow: (v: string) => void;
-  sort: string;
-  setSort: (v: string) => void;
-  filters: PlexFilters;
-  setFilters: (f: PlexFilters) => void;
-  onOpenPlaylist: (id: number) => void;
   layout: PanelLayout;
   setLayout: (l: PanelLayout) => void;
   collapsed: boolean;
@@ -42,21 +34,17 @@ const DURATION_BUCKETS: { key: string; min: number | null; max: number | null }[
 ];
 
 export default function PlexSidebar({
-  scope,
-  setScope,
-  show,
-  setShow,
-  sort,
-  setSort,
-  filters,
-  setFilters,
-  onOpenPlaylist,
   layout,
   setLayout,
   collapsed,
   onToggleCollapse,
 }: Props) {
   const { t } = useTranslation();
+  // Scope / watch-state / sort / expanded filters live in PlexProvider (shared with PlexBrowse and
+  // the header search box); this rail is the primary editor of them. Opening a playlist is a
+  // hand-off to PlexBrowse via the shared `playlistOpen` slot.
+  const { scope, show, sort, filters } = usePlex();
+  const { setScope, setShow, setSort, setFilters, setPlaylistOpen: onOpenPlaylist } = usePlexActions();
   const collFade = useScrollFade();
   const playlistsQ = useQuery({ queryKey: ["plex-playlists"], queryFn: () => api.plexPlaylists() });
   const [newPlaylist, setNewPlaylist] = useState("");