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/types/content.ts

@@ -69,6 +69,24 @@ export interface ScheduleLocation {
   days: ScheduleDay[];
 }
 
+export interface MasterClassSlot {
+  date: string;       // ISO "2026-03-13"
+  startTime: string;  // "19:00"
+  endTime: string;    // "21:00"
+}
+
+export interface MasterClassItem {
+  title: string;
+  image: string;
+  slots: MasterClassSlot[];
+  trainer: string;
+  cost: string;
+  style: string;
+  location?: string;
+  description?: string;
+  instagramUrl?: string;
+}
+
 export interface ContactInfo {
   title: string;
   addresses: string[];
@@ -113,6 +131,11 @@ export interface SiteContent {
     rentalItems: PricingItem[];
     rules: string[];
   };
+  masterClasses: {
+    title: string;
+    successMessage?: string;
+    items: MasterClassItem[];
+  };
   schedule: {
     title: string;
     locations: ScheduleLocation[];