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/game_screen.dart

@@ -150,7 +150,7 @@ class _GameScreenState extends ConsumerState<GameScreen>
     if (prev?.phase != next.phase) {
       // A finished stage ends the tutorial; otherwise the overlay would sit
       // on top of the result card and leak into the next stage.
-      if (next.phase == GamePhase.won || next.phase == GamePhase.lost) {
+      if (next.phase != GamePhase.playing) {
         ref.read(tutorialProvider.notifier).skip();
       }
       if (next.phase == GamePhase.won) {