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/asset/transfer_page.dart

@@ -152,7 +152,7 @@ class _TransferPageState extends State<TransferPage> {
         title: Text(
           '资金划转',
           style: GoogleFonts.spaceGrotesk(
-            fontSize: 18,
+            fontSize: 14,
             fontWeight: FontWeight.bold,
             color: colorScheme.onSurface,
           ),
@@ -411,14 +411,14 @@ class _TransferPageState extends State<TransferPage> {
                     FilteringTextInputFormatter.allow(RegExp(r'^\d*\.?\d{0,8}')),
                   ],
                   style: GoogleFonts.spaceGrotesk(
-                    fontSize: 32,
+                    fontSize: 18,
                     fontWeight: FontWeight.bold,
                     color: colorScheme.onSurface,
                   ),
                   decoration: InputDecoration(
                     hintText: '0.00',
                     hintStyle: GoogleFonts.spaceGrotesk(
-                      fontSize: 32,
+                      fontSize: 18,
                       fontWeight: FontWeight.bold,
                       color: colorScheme.onSurfaceVariant.withOpacity(0.3),
                     ),