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/balance_card.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 '../../../../providers/asset_provider.dart';
 import '../../../components/glass_panel.dart';
@@ -22,9 +22,6 @@ class BalanceCard extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final colorScheme = Theme.of(context).colorScheme;
-    final isDark = Theme.of(context).brightness == Brightness.dark;
-
     final displayBalance = activeTab == 0
         ? (provider.fundAccount?.balance ?? provider.overview?.fundBalance ?? '0.00')
         : _calculateTradeTotal();
@@ -38,7 +35,7 @@ class BalanceCard extends StatelessWidget {
           Text(
             'USDT 余额',
             style: AppTextStyles.bodyMedium(context).copyWith(
-              color: colorScheme.onSurfaceVariant,
+              color: context.colors.onSurfaceVariant,
               fontWeight: FontWeight.w400,
             ),
           ),
@@ -51,7 +48,7 @@ class BalanceCard extends StatelessWidget {
           Text(
             '\u2248 \$${_formatBalance(displayBalance)} USD',
             style: AppTextStyles.bodyMedium(context).copyWith(
-              color: isDark ? AppColorScheme.darkOnSurfaceMuted : colorScheme.onSurfaceVariant,
+              color: context.appColors.onSurfaceMuted,
               fontWeight: FontWeight.w400,
             ),
           ),