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/hooks/useAuth.ts

@@ -0,0 +1,31 @@
+import { useState, useEffect } from 'react';
+import { apiFetch } from '../lib/api';
+
+export interface User {
+  id: number;
+  email: string;
+  nickname: string;
+}
+
+interface AuthState {
+  user: User | null;
+  loading: boolean;
+}
+
+export function useAuth(): AuthState {
+  const [user, setUser] = useState<User | null>(null);
+  const [loading, setLoading] = useState(true);
+
+  useEffect(() => {
+    let cancelled = false;
+
+    apiFetch<User>('/profile')
+      .then((u) => { if (!cancelled) setUser(u); })
+      .catch(() => { if (!cancelled) setUser(null); })
+      .finally(() => { if (!cancelled) setLoading(false); });
+
+    return () => { cancelled = true; };
+  }, []);
+
+  return { user, loading };
+}