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/app/admin/layout.tsx

@@ -8,6 +8,7 @@ import {
   Sparkles,
   Users,
   BookOpen,
+  Star,
   Calendar,
   DollarSign,
   HelpCircle,
@@ -27,6 +28,7 @@ const NAV_ITEMS = [
   { href: "/admin/about", label: "О студии", icon: FileText },
   { href: "/admin/team", label: "Команда", icon: Users },
   { href: "/admin/classes", label: "Направления", icon: BookOpen },
+  { href: "/admin/master-classes", label: "Мастер-классы", icon: Star },
   { href: "/admin/schedule", label: "Расписание", icon: Calendar },
   { href: "/admin/pricing", label: "Цены", icon: DollarSign },
   { href: "/admin/faq", label: "FAQ", icon: HelpCircle },