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/transfer_page.dart

@@ -3,7 +3,7 @@ import 'package:flutter/services.dart';
 import 'package:provider/provider.dart';
 import 'package:lucide_icons_flutter/lucide_icons.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 '../../../data/models/account_models.dart';
@@ -76,12 +76,6 @@ class _TransferPageState extends State<TransferPage> {
   String get _fromBalance => _direction == 1 ? _fundBalance : _tradeUsdtBalance;
   String get _toBalance => _direction == 1 ? _tradeUsdtBalance : _fundBalance;
 
-  // ============================================
-  // 主题辅助
-  // ============================================
-
-  bool get _isDark => Theme.of(context).brightness == Brightness.dark;
-
   // ============================================
   // 业务逻辑
   // ============================================
@@ -363,7 +357,7 @@ class _TransferPageState extends State<TransferPage> {
                     decoration: InputDecoration(
                       hintText: '0.00',
                       hintStyle: AppTextStyles.numberLarge(context).copyWith(
-                        color: AppColorScheme.darkOnSurfaceMuted,
+                        color: context.appColors.onSurfaceMuted,
                       ),
                       border: InputBorder.none,
                       contentPadding: EdgeInsets.zero,
@@ -375,7 +369,7 @@ class _TransferPageState extends State<TransferPage> {
                   padding: const EdgeInsets.only(left: AppSpacing.sm),
                   child: Text('USDT', style: AppTextStyles.headlineMedium(context).copyWith(
                     fontWeight: FontWeight.w400,
-                    color: AppColorScheme.darkOnSurfaceMuted,
+                    color: context.appColors.onSurfaceMuted,
                   )),
                 ),
               ],
@@ -425,13 +419,13 @@ class _TransferPageState extends State<TransferPage> {
   // ============================================
 
   Widget _buildTipsCard() {
-    final upColor = AppColorScheme.getUpColor(_isDark);
+    final upColor = context.appColors.up;
 
     return Container(
       width: double.infinity,
       padding: const EdgeInsets.symmetric(horizontal: AppSpacing.md, vertical: 12),
       decoration: BoxDecoration(
-        color: AppColorScheme.getUpBackgroundColor(_isDark),
+        color: context.appColors.upBackground,
         borderRadius: BorderRadius.circular(AppRadius.lg),
       ),
       child: Row(