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

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

frontend/index.html

@@ -0,0 +1,13 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <link rel="icon" type="image/svg+xml" href="/favicon.svg" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>BrainWave</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="/src/main.tsx"></script>
+  </body>
+</html>

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>
+}

frontend/src/main.tsx

@@ -0,0 +1,11 @@
+import { StrictMode } from 'react'
+import { createRoot } from 'react-dom/client'
+import 'katex/dist/katex.min.css'
+import './index.css'
+import App from './App.tsx'
+
+createRoot(document.getElementById('root')!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+)