feat: trading terminal responsivo mobile + spread ajustado USD/BRL

Adiciona CSS responsivo para trading terminal nos breakpoints 900px e 600px.
Aplica spread corporativo nas cotações USD live (compra: -0.43%, venda: +0.05%).

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

src/admin-panel.js

@@ -496,6 +496,7 @@ document.addEventListener('keydown', (e) => {
     document.querySelectorAll('.modal-overlay.active').forEach(m => m.classList.remove('active'));
   }
 });
+
 </script>
 </body>
 </html>`;