fix(birthday): show all CMS extra items, remove hardcoded slice(3,6) limit

extraItems now uses slice(3) without end limit — items 4-N all appear
in the "Також можна додатково замовити" grid instead of being silently dropped.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/(frontend)/dni-narodzhennia/page.tsx

@@ -257,8 +257,8 @@ export default async function BirthdayPage() {
       ? cmsPackageItems.slice(0, 3).map(toCard)
       : FALLBACK_PACKAGE_ITEMS.map(toCard)
   const extraItems: PackageCardData[] =
-    cmsHasPhotos && cmsPackageItems.length >= 6
-      ? cmsPackageItems.slice(3, 6).map(toCard)
+    cmsHasPhotos && cmsPackageItems.length > 3
+      ? cmsPackageItems.slice(3).map(toCard)
       : FALLBACK_EXTRA_ITEMS.map(toCard)
 
   const whyTitle = (d?.whyTitle as string) ?? 'Чому варто святкувати у Шуміленді'