feat: add master classes section with registration system

- New master classes section on landing page with upcoming events grid
- Admin CRUD for master classes (image, slots, trainer, style, cost, location)
- User signup modal (name + Instagram required, Telegram optional)
- Admin registration management: view, add, edit, delete with quick-contact links
- Customizable success message for signup confirmation
- Auto-filter past events, Russian date formatting, duration auto-calculation

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

src/lib/constants.ts

@@ -11,6 +11,7 @@ export const NAV_LINKS: NavLink[] = [
   { label: "О нас", href: "#about" },
   { label: "Команда", href: "#team" },
   { label: "Направления", href: "#classes" },
+  { label: "Мастер-классы", href: "#master-classes" },
   { label: "Расписание", href: "#schedule" },
   { label: "Стоимость", href: "#pricing" },
   { label: "FAQ", href: "#faq" },