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>

test/state/season_flow_test.dart

@@ -77,4 +77,20 @@ void main() {
     expect(container.read(seasonFlowProvider)!.hasNext, isFalse);
     expect(container.read(gameSessionProvider)!.phase, GamePhase.playing);
   });
+
+  test('clear() resets flow to null so Classic mode has no stale season context',
+      () async {
+    final container = await _container();
+    final flow = container.read(seasonFlowProvider.notifier);
+    flow.startSeasonStage(_pack(), 0);
+
+    // State is non-null after starting a season stage.
+    expect(container.read(seasonFlowProvider), isNotNull);
+
+    flow.clear();
+
+    // After clear(), the flow must be null so GameScreen's tutorial/theme
+    // checks can't fire for a Classic (endless) session.
+    expect(container.read(seasonFlowProvider), isNull);
+  });
 }