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/orders/fund_orders_page.dart

@@ -200,7 +200,7 @@ class _FundOrdersPageState extends State<FundOrdersPage> {
               Text(
                 '${isDeposit ? '+' : '-'}${order.amount} USDT',
                 style: TextStyle(
-                  fontSize: 20,
+                  fontSize: 16,
                   fontWeight: FontWeight.bold,
                   color: isDeposit ? upColor : downColor,
                 ),