style(ui): 统一页面字号间距与状态样式

lib/pages/auth/auth_widgets.dart

@@ -103,7 +103,7 @@ class _AuthCard extends StatelessWidget {
             l10n.authSubtitle,
             style: const TextStyle(
               color: AppColors.muted,
-              fontSize: 13,
+              fontSize: AppDimens.body,
               height: 1.45,
             ),
           ),