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/playlists_list_screen.dart

@@ -51,7 +51,7 @@ class PlaylistsListScreen extends ConsumerWidget {
                 final p = items[i];
                 return _PlaylistTile(
                   playlist: p,
-                  onTap: () => ctx.push('/playlists/${p.id}'),
+                  onTap: () => ctx.push('/playlists/${p.id}', extra: p),
                 );
               },
             ),