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/main.tsx

@@ -5,6 +5,7 @@ import ErrorBoundary from "./components/ErrorBoundary";
 import { ConfirmProvider } from "./components/ConfirmProvider";
 import { NavigationProvider } from "./components/NavigationProvider";
 import { FeedFiltersProvider } from "./components/FeedFiltersProvider";
+import { PlaylistsProvider } from "./components/PlaylistsProvider";
 import "./i18n";
 import "./index.css";
 
@@ -39,7 +40,9 @@ const root =
         <ConfirmProvider>
           <NavigationProvider>
             <FeedFiltersProvider>
-              <App />
+              <PlaylistsProvider>
+                <App />
+              </PlaylistsProvider>
             </FeedFiltersProvider>
           </NavigationProvider>
         </ConfirmProvider>