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/Playlists.tsx

@@ -31,7 +31,7 @@ import {
   X,
   Youtube,
 } from "lucide-react";
-import { api, type Video } from "../lib/api";
+import { api, type Me, type Video } from "../lib/api";
 import { formatDuration } from "../lib/format";
 import { notify } from "../lib/notifications";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
@@ -40,6 +40,7 @@ const PlayerModal = lazy(() => import("./PlayerModal"));
 import UndoToolbar from "./UndoToolbar";
 import { PageToolbar } from "./PageShell";
 import { useConfirm } from "./ConfirmProvider";
+import { usePlaylists, usePlaylistsActions } from "./PlaylistsProvider";
 
 type SortKey = "manual" | "title" | "duration" | "channel";
 type SortDir = "asc" | "desc";
@@ -178,17 +179,12 @@ function Row({
   );
 }
 
-export default function Playlists({
-  canWrite,
-  selectedId,
-  setSelectedId,
-}: {
-  canWrite: boolean;
-  // The selected playlist is App state, shared with the App-level PlaylistsRail (which owns the
+export default function Playlists({ me }: { me: Me }) {
+  const canWrite = me.can_write;
+  // The selected playlist lives in PlaylistsProvider, shared with the PlaylistsRail (which owns the
   // list + its rail controls). This detail pane reacts to the selection.
-  selectedId: number | null;
-  setSelectedId: (id: number | null) => void;
-}) {
+  const { selectedId } = usePlaylists();
+  const { setSelectedId } = usePlaylistsActions();
   const { t } = useTranslation();
   const qc = useQueryClient();
   const confirm = useConfirm();