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

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

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>,
+)