fix: clear season flow when starting Classic so tutorial/theme can't leak

- Add SeasonFlowNotifier.clear() to null out the flow state
- Call clear() in HomeScreen's Classic button before startStage()
- Broaden tutorial-end guard to next.phase != GamePhase.playing (covers stuck)
- Add regression test: clear() resets flow to null

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/ui/screens/home_screen.dart

@@ -84,6 +84,7 @@ class HomeScreen extends ConsumerWidget {
                     ),
                     onPressed: () {
                       if (!(ModalRoute.of(context)?.isCurrent ?? true)) return;
+                      ref.read(seasonFlowProvider.notifier).clear();
                       ref.read(gameSessionProvider.notifier).startStage(
                             StageConfig.endless(
                               seed: DateTime.now().millisecondsSinceEpoch,