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/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)),
     );
   }
 }