refactor: 优化字号主题体系，参考成熟交易平台标准

- 重构主题字号体系 (h1-h4, body, amount等)
- 修复16个页面文件中的硬编码字号
- 新字号层级参考币安/OKX标准
- Display: 22/20/18px (总资产、价格)
- Headline: 15/14/13px (标题、副标题)
- Body: 13/12/11px (正文、辅助文字)
- Label: 11/10/9px (标签)
- Number: 22/16/13px (数字)

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

@@ -242,7 +242,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
           Text(
             item.title,
             style: TextStyle(
-              fontSize: 28,
+              fontSize: 18,
               fontWeight: FontWeight.bold,
               color: colorScheme.onSurface,
               letterSpacing: -0.5,
@@ -254,7 +254,7 @@ class _OnboardingPageState extends State<OnboardingPage> {
             item.description,
             textAlign: TextAlign.center,
             style: TextStyle(
-              fontSize: 16,
+              fontSize: 14,
               color: colorScheme.onSurfaceVariant,
               height: 1.6,
             ),