fix(portal): corporate landing SEO, favicon, and public home UX

Add sankofa.nexus marketing site with institutional grade scorecards,
server-side metadata/title, dynamic icons, favicon rewrite, and instant
landing render without session-loading flash; split authenticated AppShell
from unauthenticated corporate chrome.

Co-authored-by: Cursor <cursoragent@cursor.com>

portal/src/components/auth/PortalSignInCard.tsx

@@ -0,0 +1,61 @@
+'use client';
+
+import { signIn } from 'next-auth/react';
+import { useCallback, useState, type ReactNode } from 'react';
+
+import { CloudflareTurnstile } from '@/components/security/CloudflareTurnstile';
+
+const TURNSTILE_SITE_KEY = process.env.NEXT_PUBLIC_CLOUDFLARE_TURNSTILE_SITE_KEY;
+
+export interface PortalSignInCardProps {
+  badge?: string;
+  title: string;
+  subtitle: string;
+  callbackUrl?: string;
+  /** Extra hint under the sign-in button (e.g. dev IdP note) */
+  footer?: ReactNode;
+}
+
+export function PortalSignInCard({
+  badge = 'Sankofa Phoenix',
+  title,
+  subtitle,
+  callbackUrl = '/',
+  footer,
+}: PortalSignInCardProps) {
+  const [turnstileToken, setTurnstileToken] = useState<string | null>(() =>
+    TURNSTILE_SITE_KEY ? null : ''
+  );
+
+  const onTurnstileToken = useCallback((token: string | null) => {
+    setTurnstileToken(token);
+  }, []);
+
+  const canSignIn = !TURNSTILE_SITE_KEY || Boolean(turnstileToken);
+
+  return (
+    <div className="w-full max-w-md rounded-2xl border border-gray-800 bg-gray-900/80 p-8 shadow-xl shadow-black/40 backdrop-blur-sm">
+      <p className="mb-1 text-center text-sm font-medium uppercase tracking-wide text-orange-400">{badge}</p>
+      <h1 className="mb-2 text-center text-2xl font-bold text-white">{title}</h1>
+      <p className="mb-8 text-center text-gray-400">{subtitle}</p>
+
+      {TURNSTILE_SITE_KEY ? (
+        <div className="mb-6">
+          <p className="mb-2 text-center text-sm text-gray-500">Verification</p>
+          <CloudflareTurnstile siteKey={TURNSTILE_SITE_KEY} onToken={onTurnstileToken} />
+        </div>
+      ) : null}
+
+      <button
+        type="button"
+        disabled={!canSignIn}
+        onClick={() => signIn(undefined, { callbackUrl })}
+        className="w-full rounded-lg bg-gradient-to-r from-orange-500 to-amber-500 px-6 py-3 font-semibold text-gray-950 shadow-lg transition hover:from-orange-400 hover:to-amber-400 focus:outline-none focus:ring-2 focus:ring-orange-400 focus:ring-offset-2 focus:ring-offset-gray-900 disabled:cursor-not-allowed disabled:opacity-50"
+      >
+        Sign In
+      </button>
+
+      {footer}
+    </div>
+  );
+}