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/auth/login_page.dart

@@ -5,6 +5,7 @@ import 'package:provider/provider.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 '../../../providers/auth_provider.dart';
 import '../main/main_page.dart';
 import 'register_page.dart';
@@ -36,12 +37,8 @@ class _LoginPageState extends State<LoginPage> {
 
   @override
   Widget build(BuildContext context) {
-    final isDark = Theme.of(context).brightness == Brightness.dark;
-
     return Scaffold(
-      backgroundColor: isDark
-          ? AppColorScheme.darkBackground
-          : AppColorScheme.lightSurface,
+      backgroundColor: context.colors.surface,
       body: SafeArea(
         child: SingleChildScrollView(
           padding: const EdgeInsets.symmetric(
@@ -53,13 +50,13 @@ class _LoginPageState extends State<LoginPage> {
             child: Column(
               children: [
                 // 顶部品牌区域
-                _buildBrandSection(isDark),
+                _buildBrandSection(),
                 const SizedBox(height: AppSpacing.xxl),
                 // 表单区域
-                _buildFormSection(isDark),
+                _buildFormSection(),
                 const SizedBox(height: AppSpacing.xl),
                 // 底部注册链接
-                _buildRegisterRow(isDark),
+                _buildRegisterRow(),
               ],
             ),
           ),
@@ -72,16 +69,16 @@ class _LoginPageState extends State<LoginPage> {
   // 品牌区域 - Logo + 品牌名 + 标语
   // ============================================
 
-  Widget _buildBrandSection(bool isDark) {
+  Widget _buildBrandSection() {
     return Column(
       children: [
         // Logo 圆形：渐变 #1F2937 → #374151，内含 "M"
         Container(
           width: _logoCircleSize,
           height: _logoCircleSize,
-          decoration: BoxDecoration(
+          decoration: const BoxDecoration(
             shape: BoxShape.circle,
-            gradient: const LinearGradient(
+            gradient: LinearGradient(
               begin: Alignment.topCenter,
               end: Alignment.bottomCenter,
               colors: [AppColorScheme.darkSurfaceContainerHigh, AppColorScheme.darkOutline],
@@ -93,9 +90,7 @@ class _LoginPageState extends State<LoginPage> {
             style: AppTextStyles.displayLarge(context).copyWith(
               fontSize: 32,
               fontWeight: FontWeight.w800,
-              color: isDark
-                  ? AppColorScheme.darkOnSurface
-                  : AppColorScheme.darkOnPrimary,
+              color: context.colors.onPrimary,
             ),
           ),
         ),
@@ -105,9 +100,7 @@ class _LoginPageState extends State<LoginPage> {
           'MONISUO',
           style: AppTextStyles.displayLarge(context).copyWith(
             letterSpacing: 3,
-            color: isDark
-                ? AppColorScheme.darkOnSurface
-                : AppColorScheme.lightOnSurface,
+            color: context.colors.onSurface,
           ),
           textAlign: TextAlign.center,
         ),
@@ -116,9 +109,7 @@ class _LoginPageState extends State<LoginPage> {
         Text(
           '虚拟货币模拟交易平台',
           style: AppTextStyles.bodyLarge(context).copyWith(
-            color: isDark
-                ? AppColorScheme.darkOnSurfaceVariant
-                : AppColorScheme.lightOnSurfaceVariant,
+            color: context.colors.onSurfaceVariant,
           ),
           textAlign: TextAlign.center,
         ),
@@ -130,30 +121,20 @@ class _LoginPageState extends State<LoginPage> {
   // 表单区域 - 用户名 + 密码 + 登录按钮
   // ============================================
 
-  Widget _buildFormSection(bool isDark) {
+  Widget _buildFormSection() {
     return Column(
       crossAxisAlignment: CrossAxisAlignment.stretch,
       children: [
-        _buildUsernameField(isDark),
+        _buildUsernameField(),
         const SizedBox(height: AppSpacing.md),
-        _buildPasswordField(isDark),
+        _buildPasswordField(),
         const SizedBox(height: AppSpacing.sm),
-        _buildLoginButton(isDark),
+        _buildLoginButton(),
       ],
     );
   }
 
-  Widget _buildUsernameField(bool isDark) {
-    final borderColor = isDark
-        ? AppColorScheme.darkOutlineVariant
-        : AppColorScheme.lightOutlineVariant;
-    final cardColor = isDark
-        ? AppColorScheme.darkSurfaceContainer
-        : AppColorScheme.lightSurfaceLowest;
-    final iconColor = isDark
-        ? AppColorScheme.darkOnSurfaceMuted
-        : AppColorScheme.lightOnSurfaceMuted;
-
+  Widget _buildUsernameField() {
     return SizedBox(
       height: _inputHeight,
       child: ShadInputFormField(
@@ -161,32 +142,25 @@ class _LoginPageState extends State<LoginPage> {
         placeholder: const Text('请输入用户名'),
         leading: Padding(
           padding: const EdgeInsets.only(right: AppSpacing.sm),
-          child: Icon(LucideIcons.user, size: 18, color: iconColor),
+          child: Icon(LucideIcons.user, size: 18, color: context.appColors.onSurfaceMuted),
         ),
         validator: _validateUsername,
         controller: _usernameController,
         decoration: ShadDecoration(
           border: ShadBorder.all(
-            color: borderColor,
+            color: context.colors.outlineVariant,
             radius: AppRadius.radiusLg,
           ),
         ),
         style: AppTextStyles.headlineMedium(context).copyWith(
-          color: isDark
-              ? AppColorScheme.darkOnSurface
-              : AppColorScheme.lightOnSurface,
+          color: context.colors.onSurface,
         ),
       ),
     );
   }
 
-  Widget _buildPasswordField(bool isDark) {
-    final borderColor = isDark
-        ? AppColorScheme.darkOutlineVariant
-        : AppColorScheme.lightOutlineVariant;
-    final iconColor = isDark
-        ? AppColorScheme.darkOnSurfaceMuted
-        : AppColorScheme.lightOnSurfaceMuted;
+  Widget _buildPasswordField() {
+    final iconColor = context.appColors.onSurfaceMuted;
 
     return SizedBox(
       height: _inputHeight,
@@ -210,27 +184,21 @@ class _LoginPageState extends State<LoginPage> {
         controller: _passwordController,
         decoration: ShadDecoration(
           border: ShadBorder.all(
-            color: borderColor,
+            color: context.colors.outlineVariant,
             radius: AppRadius.radiusLg,
           ),
         ),
         style: AppTextStyles.headlineMedium(context).copyWith(
-          color: isDark
-              ? AppColorScheme.darkOnSurface
-              : AppColorScheme.lightOnSurface,
+          color: context.colors.onSurface,
         ),
       ),
     );
   }
 
-  Widget _buildLoginButton(bool isDark) {
+  Widget _buildLoginButton() {
     // 设计稿: accent-primary = light:#1F2937 / dark:#D4AF37
-    final buttonColor = isDark
-        ? AppColorScheme.darkSecondary
-        : AppColorScheme.darkSurfaceContainerHigh;
-    final textColor = isDark
-        ? AppColorScheme.darkBackground
-        : AppColorScheme.darkOnPrimary;
+    final buttonColor = context.appColors.accentPrimary;
+    final textColor = context.colors.onPrimary;
 
     return Consumer<AuthProvider>(
       builder: (context, auth, _) {
@@ -270,14 +238,10 @@ class _LoginPageState extends State<LoginPage> {
   // 底部注册链接
   // ============================================
 
-  Widget _buildRegisterRow(bool isDark) {
+  Widget _buildRegisterRow() {
     // gold-accent: light=#F59E0B / dark=#D4AF37
-    final goldColor = isDark
-        ? AppColorScheme.darkSecondary
-        : AppColorScheme.darkSecondaryFixed;
-    final secondaryTextColor = isDark
-        ? AppColorScheme.darkOnSurfaceVariant
-        : AppColorScheme.lightOnSurfaceVariant;
+    final goldColor = context.appColors.accentPrimary;
+    final secondaryTextColor = context.colors.onSurfaceVariant;
 
     return Padding(
       padding: const EdgeInsets.only(bottom: AppSpacing.xl),