feat(adminsite): session guards and the auth screens

Route-group layouts do the guarding. A customer session on /staff/* is
redirected to its own home rather than shown a refusal -- there is nothing to
tell them about. This is UX only: admin enforces the same boundary with
RequireStaff/RequireCustomer and answers 404 rather than 403 for another
account's data, which is the layer that actually matters.

Signup carries the honeypot the backend expects and reports "check your
email" rather than claiming an account exists, matching a backend that
creates nothing until the link is opened.

Buttons match site/'s .btn--solid and .btn--line, neutral border included.

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

adminsite/app/not-found.tsx

@@ -0,0 +1,15 @@
+import Link from "next/link";
+
+export default function NotFound() {
+    return (
+        <main className="mx-auto max-w-rail px-5 py-12">
+            <h1 className="text-3xl">Nothing here</h1>
+            <p className="mt-2 text-ink-2">
+                That page does not exist, or it belongs to an account you are not signed in to.
+            </p>
+            <Link href="/" className="mt-4 inline-block text-accent underline">
+                Back to your account
+            </Link>
+        </main>
+    );
+}