refactor(theme): 迁移主题感知颜色至 ThemeExtension

- 创建 AppThemeColors ThemeExtension 类，统一管理主题感知颜色（涨跌色、卡片背景、渐变等）
- 从 AppColorScheme 移除主题感知辅助函数，仅保留静态颜色常量
- 在 AppTheme 中注册 ThemeExtension，支持深色/浅色主题工厂
- 重构所有 UI 组件使用 context.appColors 访问主题颜色，替代硬编码的 AppColorScheme 方法调用
- 移除组件中重复的 isDark 判断逻辑，简化颜色获取方式
- 保持向后兼容性，所有现有功能不变

flutter_monisuo/lib/ui/pages/asset/components/holdings_section.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
 import '../../../../core/theme/app_theme.dart';
-import '../../../../core/theme/app_color_scheme.dart';
+import '../../../../core/theme/app_theme_extension.dart';
 import '../../../../core/theme/app_spacing.dart';
 import '../../../../data/models/account_models.dart';
 import '../../../components/glass_panel.dart';
@@ -116,11 +116,9 @@ class HoldingRow extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final colorScheme = Theme.of(context).colorScheme;
-    final isDark = Theme.of(context).brightness == Brightness.dark;
-    final accentColor = isDark ? colorScheme.secondary : colorScheme.primary;
+    final accentColor = context.appColors.accentPrimary;
     final accentBgColor = accentColor.withValues(alpha: 0.1);
-    final profitColor = isProfit ? AppColorScheme.getUpColor(isDark) : AppColorScheme.getDownColor(isDark);
+    final profitColor = isProfit ? context.appColors.up : context.appColors.down;
 
     return Padding(
       padding: const EdgeInsets.symmetric(horizontal: AppSpacing.md, vertical: 14),
@@ -159,7 +157,7 @@ class HoldingRow extends StatelessWidget {
                   quantity,
                   style: AppTextStyles.bodyMedium(context).copyWith(
                     fontWeight: FontWeight.w400,
-                    color: colorScheme.onSurfaceVariant,
+                    color: context.colors.onSurfaceVariant,
                   ),
                 ),
               ],