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);

lib/ui/screens/season_map_screen.dart

@@ -21,7 +21,7 @@ class SeasonMapScreen extends ConsumerWidget {
       loading: () =>
           const Scaffold(body: Center(child: CircularProgressIndicator())),
       error: (e, _) => Scaffold(body: Center(child: Text('$e'))),
-      data: (list) => _JourneyMap(pack: list.first),
+      data: (list) => _JourneyMap(pack: activeSeason(list)),
     );
   }
 }

lib/ui/screens/season_title_screen.dart

@@ -63,7 +63,7 @@ class _SeasonTitleScreenState extends ConsumerState<SeasonTitleScreen> {
           _auto?.cancel();
           _auto = Timer(const Duration(milliseconds: 1600), _go);
         }
-        final pack = list.first;
+        final pack = activeSeason(list);
         final locale = Localizations.localeOf(context).languageCode;
         final number = int.tryParse(pack.seasonId.split('_').last) ?? 1;
         return GestureDetector(