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/asset_page.dart

@@ -121,7 +121,7 @@ class _AssetCard extends StatelessWidget {
           Text(
             '\$${overview?.totalAsset ?? '0.00'}',
             style: GoogleFonts.spaceGrotesk(
-              fontSize: 36,
+              fontSize: 22,
               fontWeight: FontWeight.bold,
               color: Colors.white,
             ),
@@ -284,7 +284,7 @@ class _FundAccountCard extends StatelessWidget {
           Text(
             displayBalance,
             style: GoogleFonts.spaceGrotesk(
-              fontSize: 28,
+              fontSize: 20,
               fontWeight: FontWeight.bold,
               color: colorScheme.onSurface,
             ),
@@ -413,7 +413,7 @@ class _TradeAccountCard extends StatelessWidget {
           Text(
             totalValue.toStringAsFixed(2),
             style: GoogleFonts.spaceGrotesk(
-              fontSize: 28,
+              fontSize: 20,
               fontWeight: FontWeight.bold,
               color: colorScheme.onSurface,
             ),
@@ -592,7 +592,7 @@ void _showDepositDialog(BuildContext context) {
                     Text(
                       'Deposit (充值)',
                       style: GoogleFonts.spaceGrotesk(
-                        fontSize: 24,
+                        fontSize: 16,
                         fontWeight: FontWeight.bold,
                         color: colorScheme.onSurface,
                       ),
@@ -708,7 +708,7 @@ void _showDepositResultDialog(BuildContext context, Map<String, dynamic> data) {
                 Text(
                   '充值申请成功',
                   style: GoogleFonts.spaceGrotesk(
-                    fontSize: 20,
+                    fontSize: 16,
                     fontWeight: FontWeight.bold,
                     color: colorScheme.onSurface,
                   ),
@@ -938,7 +938,7 @@ void _showWithdrawDialog(BuildContext context, String? balance) {
                   Text(
                     '提现 (Withdraw)',
                     style: GoogleFonts.spaceGrotesk(
-                      fontSize: 20,
+                      fontSize: 16,
                       fontWeight: FontWeight.bold,
                       color: colorScheme.onSurface,
                     ),
@@ -1117,7 +1117,7 @@ void _showResultDialog(BuildContext context, String title, String? message) {
           children: [
             Text(title,
                 style: GoogleFonts.spaceGrotesk(
-                  fontSize: 20,
+                  fontSize: 16,
                   fontWeight: FontWeight.bold,
                   color: colorScheme.onSurface,
                 )),
@@ -1174,7 +1174,7 @@ void _showKycRequiredDialog(BuildContext context) {
             Text(
               '需要实名认证',
               style: GoogleFonts.spaceGrotesk(
-                fontSize: 20,
+                fontSize: 16,
                 fontWeight: FontWeight.bold,
                 color: colorScheme.onSurface,
               ),

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),
                     ),