fix(flutter): 回退底部导航为四槽

lib/core/theme/app_colors.dart

@@ -42,11 +42,6 @@ abstract final class AppColors {
   /// 底部导航底色。
   static const bar = Color(0xFFFFFDFB);
 
-  /// 中央操作按钮渐变。
-  static const navCenterStart = Color(0xFFFF9A3D);
-  static const navCenterEnd = Color(0xFFFF6A00);
-  static const navCenterShadow = Color(0x33FF7A1A);
-
   /// 卡片轻阴影。
   static const cardShadow = Color(0x14FF8A3D);
 

lib/core/theme/app_dimens.dart

@@ -47,8 +47,6 @@ abstract final class AppDimens {
 
   static const double navHeight = 64;
   static const double navIcon = 20;
-  static const double navCenter = 56;
-  static const double navCenterIcon = 28;
   static const double progressHeight = 3;
   static const double progressRadius = 2;
   static const double progressLineHeight = 3;