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/trade/trade_page.dart

@@ -1,8 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:shadcn_ui/shadcn_ui.dart';
 import 'package:provider/provider.dart';
-import '../../../core/theme/app_color_scheme.dart';
 import '../../../core/theme/app_spacing.dart';
+import '../../../core/theme/app_theme_extension.dart';
 import '../../../data/models/coin.dart';
 import '../../../providers/market_provider.dart';
 import '../../../providers/asset_provider.dart';
@@ -113,10 +113,9 @@ class _TradePageState extends State<TradePage>
   @override
   Widget build(BuildContext context) {
     super.build(context);
-    final colorScheme = Theme.of(context).colorScheme;
 
     return Scaffold(
-      backgroundColor: colorScheme.background,
+      backgroundColor: context.colors.background,
       body: Consumer2<MarketProvider, AssetProvider>(
         builder: (context, market, asset, _) {
           return SafeArea(
@@ -150,7 +149,6 @@ class _TradePageState extends State<TradePage>
                   else
                     PlaceholderCard(
                       message: '请先选择交易币种',
-                      colorScheme: colorScheme,
                     ),
                   const SizedBox(height: AppSpacing.md),
 
@@ -262,8 +260,6 @@ class _TradePageState extends State<TradePage>
   }
 
   void _showResultDialog(bool success, String title, String message) {
-    final colorScheme = Theme.of(context).colorScheme;
-    final isDark = Theme.of(context).brightness == Brightness.dark;
     showShadDialog(
       context: context,
       builder: (ctx) => ShadDialog.alert(
@@ -272,8 +268,8 @@ class _TradePageState extends State<TradePage>
             NeonIcon(
               icon: success ? Icons.check_circle : Icons.error,
               color: success
-                  ? AppColorScheme.getUpColor(isDark)
-                  : colorScheme.error,
+                  ? ctx.appColors.up
+                  : ctx.colors.error,
               size: 24,
             ),
             SizedBox(width: AppSpacing.sm),