fix: journey map season-complete styling, scroll callback guard, palette constant

- Guard addPostFrameCallback with !_autoScrolled so it only fires once per
  widget lifetime instead of on every LayoutBuilder rebuild.
- Derive seasonComplete flag; pass it into _node so the last stage uses
  gold "done" styling (not "current/next") when the season is fully 3-starred.
- Extract const Color(0xFF232B4A) to GamePalette.lockedNode.
- Remove warnIfMissed: false from tap call in season_map_screen_test; ensureVisible
  already guarantees hit-testing succeeds (confirmed: test still passes cleanly).

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

test/ui/season_map_screen_test.dart

@@ -89,10 +89,7 @@ void main() {
     // Stage 1 is starred, so stage 2 (index 1) is unlocked and playable.
     // Ensure the node is visible before tapping.
     await tester.ensureVisible(find.byKey(const Key('stage_node_1')));
-    await tester.tap(
-      find.byKey(const Key('stage_node_1')),
-      warnIfMissed: false,
-    );
+    await tester.tap(find.byKey(const Key('stage_node_1')));
     await tester.pumpAndSettle();
 
     expect(find.byType(GameScreen), findsOneWidget);