feat: session content sync trigger and newest-season selection

Add seasonRefreshProvider (once-per-session FutureProvider) and activeSeason()
helper; HomeScreen listens and invalidates seasonsProvider when new packs arrive;
season_map_screen and season_title_screen switch from list.first to activeSeason.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lib/ui/screens/home_screen.dart

@@ -21,6 +21,11 @@ class HomeScreen extends ConsumerWidget {
 
   @override
   Widget build(BuildContext context, WidgetRef ref) {
+    ref.listen(seasonRefreshProvider, (_, next) {
+      if (next is AsyncData<bool> && next.value == true) {
+        ref.invalidate(seasonsProvider);
+      }
+    });
     final l10n = AppLocalizations.of(context)!;
     final streak = ref.watch(streakProvider);
     final best = ref.watch(endlessBestProvider);