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/shared/routing.dart

@@ -9,6 +9,7 @@ import '../library/album_detail_screen.dart';
 import '../library/artist_detail_screen.dart';
 import '../models/album.dart';
 import '../models/artist.dart';
+import '../models/playlist.dart';
 import '../discover/discover_screen.dart';
 import '../library/home_screen.dart';
 import '../library/library_screen.dart';
@@ -120,7 +121,10 @@ GoRouter buildRouter(Ref ref) {
           GoRoute(path: '/playlists', builder: (_, __) => const PlaylistsListScreen()),
           GoRoute(
             path: '/playlists/:id',
-            builder: (_, s) => PlaylistDetailScreen(id: s.pathParameters['id']!),
+            builder: (_, s) => PlaylistDetailScreen(
+              id: s.pathParameters['id']!,
+              seed: s.extra is Playlist ? s.extra as Playlist : null,
+            ),
           ),
           GoRoute(path: '/requests',         builder: (_, __) => const RequestsScreen()),
           GoRoute(path: '/admin',            builder: (_, __) => const AdminLandingScreen()),