fix(contribution): 恢复静态颜色常量避免编译错误

保留原有浅色模式配色（_grayText, _darkText, _bgGray, _lightGray），
页面背景已支持深色模式切换

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mining-app/lib/presentation/pages/contribution/contribution_page.dart

@@ -16,6 +16,12 @@ class ContributionPage extends ConsumerWidget {
   static const Color _orange = AppColors.orange;
   static const Color _green = AppColors.primary;
 
+  // 浅色模式设计色彩（保持原有配色）
+  static const Color _grayText = Color(0xFF6B7280);
+  static const Color _darkText = Color(0xFF1F2937);
+  static const Color _bgGray = Color(0xFFF3F4F6);
+  static const Color _lightGray = Color(0xFFF9FAFB);
+
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final user = ref.watch(userNotifierProvider);