fix(portal): NextAuth redirect loop and production NEXTAUTH_URL docs

- Remove pages.signIn pointed at API route; normalize redirects for LAN callbacks
- signIn callbackUrl /; auth error page Try Again to /
- Add .env.example; README documents public NEXTAUTH_URL (sankofa.nexus)

Made-with: Cursor

portal/src/app/api/auth/error/page.tsx

@@ -35,7 +35,7 @@ function AuthErrorContent() {
             Go Home
           </Link>
           <Link
-            href="/api/auth/signin"
+            href="/"
             className="px-6 py-3 bg-gray-600 text-white rounded-lg hover:bg-gray-700 transition-colors inline-block"
           >
             Try Again