feat(flutter): playlist detail nav hydration — header renders before fetch

Same pattern as album + artist detail. PlaylistDetailScreen accepts
optional Playlist seed via go_router extra. While
playlistDetailProvider is still resolving, render the header
(description if any + track count) plus a "loading tracks…" inline
spinner instead of an opaque full-screen CircularProgressIndicator.
The body fills in when tracks arrive via drift watch re-emit.

Routing reads s.extra as Playlist. PlaylistCard +
PlaylistsListScreen pass the playlist via extra. Surfaces with no
seed available (deep links etc.) fall back to the original full-
screen spinner.

Track row rendering already uses ListView.builder so visible row
count was never the bottleneck — the wait was for the detail fetch
itself. Header-on-tap is the win that makes the screen feel snappy.

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -21,7 +21,8 @@ class PlaylistCard extends StatelessWidget {
       child: Material(
         color: Colors.transparent,
         child: InkWell(
-          onTap: () => context.push('/playlists/${playlist.id}'),
+          onTap: () =>
+              context.push('/playlists/${playlist.id}', extra: playlist),
           child: Padding(
             padding: const EdgeInsets.symmetric(horizontal: 8),
             child: Column(crossAxisAlignment: CrossAxisAlignment.start, children: [