refactor: 批量替换 shadcn_ui 为 Material Design 组件

## 样式主题重点优化

### 颜色映射（注重主题一致性）
- mutedForeground → onSurfaceVariant
- border → outline
- card → surfaceContainer
- destructive → error
- 保留所有 AppColorScheme 自定义颜色

### 文本样式映射
- theme.textTheme.h1/muted/large → AppTextStyles.xxx(context)
- 统一使用项目定义的文本样式系统

### 组件替换（20个文件）
- ShadApp → MaterialApp（移除 ShadThemeData）
- ShadButton → ElevatedButton/OutlinedButton
- ShadDialog → AlertDialog
- ShadInputFormField → MaterialInput
- ShadSelect → DropdownButtonFormField
- ShadCard → Card
- showShadDialog → showDialog

### 依赖变更
- 移除：shadcn_ui: ^0.52.1
- 添加：lucide_icons_flutter: ^2.0.0

### 业务逻辑保护
✅ 所有 onPressed/onChanged/validator 回调保持不变
✅ 所有 controller/focusNode 数据绑定保持不变
✅ 所有布局结构（Column/Row/Padding）保持不变
✅ 仅替换 UI 组件层，业务逻辑完全保留

flutter_monisuo/lib/ui/pages/onboarding/onboarding_page.dart

@@ -1,9 +1,8 @@
 import 'package:flutter/material.dart';
-import 'package:shadcn_ui/shadcn_ui.dart';
+import 'package:lucide_icons_flutter/lucide_icons.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';
 
 /// 引導頁數據模型
@@ -93,7 +92,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(
-      backgroundColor: context.colors.surface,
+      backgroundColor: Theme.of(context).colorScheme.surface,
       body: SafeArea(
         child: Column(
           children: [
@@ -111,7 +110,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                     child: Text(
                       '跳過',
                       style: AppTextStyles.headlineMedium(context).copyWith(
-                        color: context.colors.onSurfaceVariant,
+                        color: Theme.of(context).colorScheme.onSurfaceVariant,
                       ),
                     ),
                   ),
@@ -159,8 +158,8 @@ class _OnboardingPageState extends State<OnboardingPage> {
                     child: ElevatedButton(
                       onPressed: _nextPage,
                       style: ElevatedButton.styleFrom(
-                        backgroundColor: context.colors.primary,
-                        foregroundColor: context.colors.onPrimary,
+                        backgroundColor: Theme.of(context).colorScheme.primary,
+                        foregroundColor: Theme.of(context).colorScheme.onPrimary,
                         shape: RoundedRectangleBorder(
                           borderRadius: BorderRadius.circular(AppRadius.lg),
                         ),
@@ -221,7 +220,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                           return Icon(
                             item.icon ?? LucideIcons.image,
                             size: 72,
-                            color: context.colors.onPrimary,
+                            color: Theme.of(context).colorScheme.onPrimary,
                           );
                         },
                       ),
@@ -229,7 +228,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
                   : Icon(
                       item.icon ?? LucideIcons.star,
                       size: 72,
-                      color: context.colors.onPrimary,
+                      color: Theme.of(context).colorScheme.onPrimary,
                     ),
             ),
           ),
@@ -239,7 +238,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
             item.title,
             style: AppTextStyles.displaySmall(context).copyWith(
               fontWeight: FontWeight.bold,
-              color: context.colors.onSurface,
+              color: Theme.of(context).colorScheme.onSurface,
               letterSpacing: -0.5,
             ),
           ),
@@ -249,7 +248,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
             item.description,
             textAlign: TextAlign.center,
             style: AppTextStyles.headlineMedium(context).copyWith(
-              color: context.colors.onSurfaceVariant,
+              color: Theme.of(context).colorScheme.onSurfaceVariant,
               height: 1.6,
             ),
           ),
@@ -267,7 +266,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
       width: isActive ? 24 : 8,
       height: 8,
       decoration: BoxDecoration(
-        color: isActive ? context.colors.primary : context.colors.outlineVariant,
+        color: isActive ? Theme.of(context).colorScheme.primary : Theme.of(context).colorScheme.outlineVariant,
         borderRadius: BorderRadius.circular(AppRadius.sm),
       ),
     );