feat(frontend): scaffold Tailwind design system + routing + auth callback

- Tailwind v3 + PostCSS + autoprefixer
- BrowserRouter with Layout shell (Header, theme toggle dark/light)
- Pages: HomePage, CallbackPage (SuperOAuth callback handler)
- hooks/useAuth.ts + lib/api.ts (API client base)
- styles/index.css (Tailwind directives)
- Theme persisted in localStorage (od-theme)

frontend/src/components/layout/Layout.tsx

@@ -0,0 +1,25 @@
+import { Outlet } from 'react-router-dom';
+import Header from './Header';
+import { useAuth } from '../../hooks/useAuth';
+
+interface LayoutProps {
+  theme: 'dark' | 'light';
+  onToggleTheme: () => void;
+}
+
+export default function Layout({ theme, onToggleTheme }: LayoutProps) {
+  const { user, loading } = useAuth();
+
+  return (
+    <div className="min-h-screen bg-od-bg text-od-text">
+      <Header
+        theme={theme}
+        onToggleTheme={onToggleTheme}
+        user={loading ? null : user}
+      />
+      <main className="mx-auto max-w-5xl px-4 py-8">
+        <Outlet />
+      </main>
+    </div>
+  );
+}