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>

lib/ui/theme/palette.dart

@@ -22,6 +22,7 @@ class GamePalette {
 
   static Color tile(int colorId) => tileColors[colorId % tileColors.length];
 
+  static const lockedNode = Color(0xFF232B4A);
   static const gem = Color(0xFF7CF5FF);
   static const ghostLegal = Color(0x66FFFFFF);
   static const ghostIllegal = Color(0x55FF5252);