refactor(playlists): scope module state in PlaylistsProvider

Lift the Playlists module's shared local state (rail name filter + selected
playlist) out of App into a split-context PlaylistsProvider, mirroring
FeedFiltersProvider. Header, PlaylistsRail, and the Playlists detail pane read
it via usePlaylists()/usePlaylistsActions() instead of props drilled through
App; the modules take only a uniform `me` prop now. App loses two useStates and
the selected-playlist persistence helper. (Platform S6b.2)

frontend/src/components/Header.tsx

@@ -8,6 +8,7 @@ import SearchBar from "./SearchBar";
 import SyncStatus from "./SyncStatus";
 import { useNavigation, useNavigationActions } from "./NavigationProvider";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
+import { usePlaylists, usePlaylistsActions } from "./PlaylistsProvider";
 
 // The floating top header: a row of glass pills over the content — a ModuleName pill (label +
 // accent dot + back/forward arrows), the module's SearchBar where it has one, and the per-user
@@ -18,8 +19,6 @@ export default function Header({
   setPlexQ,
   channelsSearch,
   setChannelsSearch,
-  playlistsSearch,
-  setPlaylistsSearch,
   onYtSearch,
   onGoToFullHistory,
 }: {
@@ -29,8 +28,6 @@ export default function Header({
   setPlexQ: (q: string) => void;
   channelsSearch: string;
   setChannelsSearch: (q: string) => void;
-  playlistsSearch: string;
-  setPlaylistsSearch: (q: string) => void;
   // Trigger a live YouTube search for the current term (feed only; hidden for the demo account,
   // which can't spend the shared quota).
   onYtSearch: (q: string) => void;
@@ -42,6 +39,9 @@ export default function Header({
   const { setPage } = useNavigationActions();
   const filters = useFeedFilters();
   const { setFilters } = useFeedFiltersActions();
+  // The playlists name filter lives in PlaylistsProvider (shared with its rail + detail pane).
+  const { search: playlistsSearch } = usePlaylists();
+  const { setSearch: setPlaylistsSearch } = usePlaylistsActions();
   // The ◀/▶ arrows step cyclically through the user's available modules (derived from `me`, so
   // it stays correct as modules are added/gated). Labels of every active module feed ModuleName's
   // dynamic width so the pill is sized to the widest reachable title in the current language.