feat: add booking management, Open Day, unified signup modal

- MC registrations: notification toggles (confirm/remind) with urgency
- Group bookings: save to DB from BookingModal, admin CRUD at /admin/bookings
- Open Day: full event system with schedule grid (halls × time), per-class
  booking, discount pricing (30 BYN / 20 BYN from 3+), auto-cancel threshold
- Unified SignupModal replaces 3 separate forms — consistent fields
  (name, phone, instagram, telegram), Instagram DM fallback on network error
- Centralized /admin/bookings page with 3 tabs (classes, MC, Open Day),
  collapsible sections, notification toggles, filter chips
- Unread booking badge on sidebar + dashboard widget with per-type breakdown
- Pricing: contact hint (Instagram/Telegram/phone) on price & rental tabs,
  admin toggle to show/hide
- DB migrations 5-7: group_bookings table, open_day tables, unified fields

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/app/page.tsx

@@ -12,15 +12,19 @@ import { BackToTop } from "@/components/ui/BackToTop";
 import { Header } from "@/components/layout/Header";
 import { Footer } from "@/components/layout/Footer";
 import { getContent } from "@/lib/content";
+import { OpenDay } from "@/components/sections/OpenDay";
+import { getActiveOpenDay } from "@/lib/openDay";
 
 export default function HomePage() {
   const content = getContent();
+  const openDayData = getActiveOpenDay();
 
   return (
     <>
       <Header />
       <main>
         <Hero data={content.hero} />
+        {openDayData && <OpenDay data={openDayData} />}
         <About
           data={content.about}
           stats={{