fix: BB admin login no longer bounces to distribute

Problem: /admin/* pages redirect unauthed users to /login or /account,
both of which then hit middleware redirecting to distribute.relevant
mediaproperties.com. Distribute is on a different domain, so its session
cookie doesn't carry to broadcastbeat.com — sign-in there left admins
stranded on distribute's dashboard.

Fix:
  - Remove /login (and /register) from middleware REDIRECT_PATHS. wp-login,
    wp-admin, client-login still bounce to distribute.
  - /login is now a real BB sign-in form (was a redirect to /client-login).
  - Admin pages preserve return path via /login?next=<encoded path>.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/admin/newsletter/page.tsx

@@ -101,7 +101,7 @@ export default function AdminNewsletterPage() {
   const [appliedTemplate, setAppliedTemplate] = useState<NewsletterTemplate | null>(null);
 
   useEffect(() => {
-    if (!loading && !user) router.replace('/account');
+    if (!loading && !user) router.replace('/login?next=' + encodeURIComponent(window.location.pathname));
   }, [user, loading, router]);
 
   // Handle ?template= query param (from templates page "Use" button)