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/lib/openDay.ts

@@ -0,0 +1,11 @@
+import { getActiveOpenDayEvent, getOpenDayClasses } from "@/lib/db";
+import type { OpenDayEvent, OpenDayClass } from "@/lib/db";
+
+export type { OpenDayEvent, OpenDayClass };
+
+export function getActiveOpenDay(): { event: OpenDayEvent; classes: OpenDayClass[] } | null {
+  const event = getActiveOpenDayEvent();
+  if (!event) return null;
+  const classes = getOpenDayClasses(event.id);
+  return { event, classes };
+}