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/onboarding/onboarding_page.dart

@@ -3,6 +3,7 @@ import 'package:shadcn_ui/shadcn_ui.dart';
 import '../../../core/theme/app_color_scheme.dart';
 import '../../../core/theme/app_spacing.dart';
 import '../../../core/theme/app_theme.dart';
+import '../../../core/theme/app_theme_extension.dart';
 import '../../../core/storage/local_storage.dart';
 
 /// 引导页数据模型
@@ -91,11 +92,8 @@ class _OnboardingPageState extends State<OnboardingPage> {
 
   @override
   Widget build(BuildContext context) {
-    final colorScheme = Theme.of(context).colorScheme;
-    final isDark = Theme.of(context).brightness == Brightness.dark;
-
     return Scaffold(
-      backgroundColor: colorScheme.surface,
+      backgroundColor: context.colors.surface,
       body: SafeArea(
         child: Column(
           children: [
@@ -113,7 +111,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                     child: Text(
                       '跳过',
                       style: AppTextStyles.headlineMedium(context).copyWith(
-                        color: colorScheme.onSurfaceVariant,
+                        color: context.colors.onSurfaceVariant,
                       ),
                     ),
                   ),
@@ -131,7 +129,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                 },
                 itemCount: _items.length,
                 itemBuilder: (context, index) {
-                  return _buildPage(_items[index], isDark);
+                  return _buildPage(_items[index]);
                 },
               ),
             ),
@@ -150,7 +148,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                     mainAxisAlignment: MainAxisAlignment.center,
                     children: List.generate(
                       _items.length,
-                      (index) => _buildIndicator(index, isDark),
+                      (index) => _buildIndicator(index),
                     ),
                   ),
                   const SizedBox(height: AppSpacing.xl),
@@ -161,8 +159,8 @@ class _OnboardingPageState extends State<OnboardingPage> {
                     child: ElevatedButton(
                       onPressed: _nextPage,
                       style: ElevatedButton.styleFrom(
-                        backgroundColor: colorScheme.primary,
-                        foregroundColor: colorScheme.onPrimary,
+                        backgroundColor: context.colors.primary,
+                        foregroundColor: context.colors.onPrimary,
                         shape: RoundedRectangleBorder(
                           borderRadius: BorderRadius.circular(AppRadius.lg),
                         ),
@@ -185,9 +183,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
     );
   }
 
-  Widget _buildPage(_OnboardingItem item, bool isDark) {
-    final colorScheme = Theme.of(context).colorScheme;
-
+  Widget _buildPage(_OnboardingItem item) {
     return Padding(
       padding: const EdgeInsets.symmetric(horizontal: AppSpacing.xl),
       child: Column(
@@ -225,7 +221,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                           return Icon(
                             item.icon ?? LucideIcons.image,
                             size: 72,
-                            color: AppColorScheme.darkOnPrimary,
+                            color: context.colors.onPrimary,
                           );
                         },
                       ),
@@ -233,7 +229,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                   : Icon(
                       item.icon ?? LucideIcons.star,
                       size: 72,
-                      color: AppColorScheme.darkOnPrimary,
+                      color: context.colors.onPrimary,
                     ),
             ),
           ),
@@ -243,7 +239,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
             item.title,
             style: AppTextStyles.displaySmall(context).copyWith(
               fontWeight: FontWeight.bold,
-              color: colorScheme.onSurface,
+              color: context.colors.onSurface,
               letterSpacing: -0.5,
             ),
           ),
@@ -253,7 +249,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
             item.description,
             textAlign: TextAlign.center,
             style: AppTextStyles.headlineMedium(context).copyWith(
-              color: colorScheme.onSurfaceVariant,
+              color: context.colors.onSurfaceVariant,
               height: 1.6,
             ),
           ),
@@ -262,8 +258,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
     );
   }
 
-  Widget _buildIndicator(int index, bool isDark) {
-    final colorScheme = Theme.of(context).colorScheme;
+  Widget _buildIndicator(int index) {
     final isActive = index == _currentPage;
 
     return AnimatedContainer(
@@ -272,7 +267,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
       width: isActive ? 24 : 8,
       height: 8,
       decoration: BoxDecoration(
-        color: isActive ? colorScheme.primary : colorScheme.outlineVariant,
+        color: isActive ? context.colors.primary : context.colors.outlineVariant,
         borderRadius: BorderRadius.circular(AppRadius.sm),
       ),
     );