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/data/seed.ts

@@ -41,6 +41,7 @@ const sectionData: Record<string, unknown> = {
   hero: siteContent.hero,
   about: siteContent.about,
   classes: siteContent.classes,
+  masterClasses: siteContent.masterClasses,
   faq: siteContent.faq,
   pricing: siteContent.pricing,
   schedule: siteContent.schedule,