Add public DS-2 welcome page on root for unauthenticated visitors

Second-brain scheme (logo + six cards with rays) reworked from the old
platform's infographic. Root stays role-redirecting for signed-in users;
middleware opens "/" as an exact match only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/page.tsx

@@ -1,12 +1,13 @@
 import { redirect } from "next/navigation";
 import { headers } from "next/headers";
 import { auth } from "@/lib/auth";
+import { WelcomePage } from "@/components/welcome/welcome-page";
 
 export default async function HomePage() {
   const session = await auth.api.getSession({ headers: await headers() });
 
   if (!session) {
-    redirect("/login");
+    return <WelcomePage />;
   }
 
   const role = session.user.role;