fix: 分站充值页实付金额符号从 $ 改为 ¥

实付金额是后端通过 amount × 汇率 计算的人民币值，应显示 ¥ 而非 $。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/pages/Topup.jsx

@@ -399,7 +399,7 @@ export default function Topup() {
               <p className="text-xs text-page-muted mt-2">{t('topup.calculating')}</p>
             ) : payAmount ? (
               <p className="text-xs text-page-secondary mt-2">
-                {t('topup.payAmountLabel')}: <span className="text-green-400 font-medium">${payAmount}</span>
+                {t('topup.payAmountLabel')}: <span className="text-green-400 font-medium">¥{payAmount}</span>
               </p>
             ) : null}
           </div>