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/components/gradient_button.dart

@@ -1,6 +1,6 @@
 import 'package:flutter/material.dart';
-import '../../core/theme/app_color_scheme.dart';
 import '../../core/theme/app_spacing.dart';
+import '../../core/theme/app_theme_extension.dart';
 
 /// 渐变按钮组件 - 支持 CTA 渐变效果
 ///
@@ -36,7 +36,6 @@ class GradientButton extends StatelessWidget {
     bool isLoading = false,
     bool isFullWidth = true,
     IconData? icon,
-    bool isDark = true,
   }) {
     return GradientButton(
       key: key,
@@ -45,7 +44,6 @@ class GradientButton extends StatelessWidget {
       isLoading: isLoading,
       isFullWidth: isFullWidth,
       icon: icon,
-      gradient: isDark ? AppColorScheme.darkCtaGradient : AppColorScheme.lightCtaGradient,
     );
   }
 
@@ -57,6 +55,7 @@ class GradientButton extends StatelessWidget {
     bool isLoading = false,
     bool isFullWidth = true,
     IconData? icon,
+    LinearGradient? gradient,
   }) {
     return GradientButton(
       key: key,
@@ -65,7 +64,6 @@ class GradientButton extends StatelessWidget {
       isLoading: isLoading,
       isFullWidth: isFullWidth,
       icon: icon ?? Icons.arrow_downward_rounded,
-      gradient: AppColorScheme.buyGradient,
     );
   }
 
@@ -77,6 +75,7 @@ class GradientButton extends StatelessWidget {
     bool isLoading = false,
     bool isFullWidth = true,
     IconData? icon,
+    LinearGradient? gradient,
   }) {
     return GradientButton(
       key: key,
@@ -85,16 +84,14 @@ class GradientButton extends StatelessWidget {
       isLoading: isLoading,
       isFullWidth: isFullWidth,
       icon: icon ?? Icons.arrow_upward_rounded,
-      gradient: AppColorScheme.sellGradient,
     );
   }
 
   @override
   Widget build(BuildContext context) {
-    final isDark = Theme.of(context).brightness == Brightness.dark;
-    final colorScheme = Theme.of(context).colorScheme;
-    final buttonGradient = gradient ??
-        (isDark ? AppColorScheme.darkCtaGradient : AppColorScheme.lightCtaGradient);
+    final appColors = context.appColors;
+    final colorScheme = context.colors;
+    final buttonGradient = gradient ?? appColors.ctaGradient;
 
     // 主题感知颜色 - 在渐变背景上使用 onPrimary
     final textColor = colorScheme.onPrimary;
@@ -172,11 +169,8 @@ class GhostButton extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final isDark = Theme.of(context).brightness == Brightness.dark;
-    final textColor = isDark ? AppColorScheme.darkPrimary : AppColorScheme.lightPrimary;
-    final borderColor = isDark
-        ? AppColorScheme.darkOutlineVariant.withValues(alpha: 0.15)
-        : AppColorScheme.lightOutlineVariant.withValues(alpha: 0.3);
+    final textColor = context.colors.primary;
+    final borderColor = context.appColors.ghostBorder;
 
     return Container(
       width: isFullWidth ? double.infinity : null,