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/trade/trade_page.dart

@@ -331,7 +331,7 @@ class _ConfirmDialog extends StatelessWidget {
               child: Text(
                 '确认${isBuy ? '买入' : '卖出'}',
                 style: GoogleFonts.spaceGrotesk(
-                  fontSize: 20,
+                  fontSize: 16,
                   fontWeight: FontWeight.bold,
                   color: colorScheme.onSurface,
                 ),
@@ -486,7 +486,7 @@ class _CoinSelector extends StatelessWidget {
                 children: [
                   Text('选择币种',
                       style: GoogleFonts.spaceGrotesk(
-                        fontSize: 20,
+                        fontSize: 16,
                         fontWeight: FontWeight.bold,
                         color: colorScheme.onSurface,
                       )),
@@ -679,7 +679,7 @@ class _PriceCard extends StatelessWidget {
                 Text(
                   '\$${coin.formattedPrice}',
                   style: GoogleFonts.spaceGrotesk(
-                    fontSize: 30,
+                    fontSize: 20,
                     fontWeight: FontWeight.bold,
                     color: colorScheme.onSurface,
                   ),
@@ -1138,7 +1138,7 @@ class _AmountInputState extends State<_AmountInput> {
             keyboardType: const TextInputType.numberWithOptions(decimal: true),
             onChanged: (_) => _checkLimit(),
             style: GoogleFonts.spaceGrotesk(
-              fontSize: 22,
+              fontSize: 18,
               fontWeight: FontWeight.bold,
               color: colorScheme.onSurface,
             ),