feat: port design system CSS, update entry point and page title

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

frontend/src/App.tsx

@@ -0,0 +1,3 @@
+export default function App() {
+  return <div className="shell"><header className="topbar"><div className="brand"><span className="brand-mark" /><span>BrainWave</span></div></header></div>
+}