feat: add news section with admin editor and public display

- NewsItem type with title, text, date, optional image and link
- Admin page at /admin/news with image upload and auto-date
- Public section between Pricing and FAQ, hidden when empty
- Nav link auto-hides when no news items exist

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

src/lib/constants.ts

@@ -15,5 +15,6 @@ export const NAV_LINKS: NavLink[] = [
   { label: "Расписание", href: "#schedule" },
   { label: "Стоимость", href: "#pricing" },
   { label: "FAQ", href: "#faq" },
+  { label: "Новости", href: "#news" },
   { label: "Контакты", href: "#contact" },
 ];