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/middleware.ts

@@ -14,9 +14,11 @@ export function middleware(request: NextRequest) {
   }
 
   if (
+    pathname === "/" || // public welcome page; "/" must stay an EXACT match (startsWith would open everything)
     PUBLIC_ROUTES.some((route) => pathname.startsWith(route)) ||
     pathname.startsWith("/_next") ||
-    pathname.startsWith("/favicon")
+    pathname.startsWith("/favicon") ||
+    pathname === "/logo.svg"
   ) {
     return NextResponse.next();
   }