fix(flutter): 回退底部导航为四槽

test/theme/app_dimens_test.dart

@@ -21,7 +21,6 @@ void main() {
     expect(AppDimens.primaryButtonHeight, 52);
     expect(AppDimens.tapTarget, 44);
     expect(AppDimens.navHeight, 64);
-    expect(AppDimens.navCenter, 56);
     expect(AppDimens.progressHeight, 3);
   });
 }