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/components/asset_card.dart

@@ -104,7 +104,7 @@ class AssetCard extends StatelessWidget {
               style: theme.textTheme.h1.copyWith(
                 fontWeight: FontWeight.bold,
                 color: primaryTextColor,
-                fontSize: 32,
+                fontSize: 20,
               ),
             ),
             // 盈亏信息

flutter_monisuo/lib/ui/components/coin_card.dart

@@ -51,7 +51,7 @@ class CoinCard extends StatelessWidget {
                 child: Text(
                   icon ?? code.substring(0, 1),
                   style: TextStyle(
-                    fontSize: 20,
+                    fontSize: 16,
                     color: theme.colorScheme.primary,
                     fontWeight: FontWeight.bold,
                   ),