feat: centralize popup texts in new admin tab /admin/popups

- New admin page for shared popup texts (success, waiting list, error, Instagram hint)
- Removed popup fields from MC and Open Day admin editors
- All SignupModals now read from centralized popups config
- Stored as "popups" section in DB with fallback defaults

src/app/admin/layout.tsx

@@ -23,6 +23,7 @@ import {
   ChevronLeft,
   ClipboardList,
   DoorOpen,
+  MessageSquare,
 } from "lucide-react";
 
 const NAV_ITEMS = [
@@ -39,6 +40,7 @@ const NAV_ITEMS = [
   { href: "/admin/pricing", label: "Цены", icon: DollarSign },
   { href: "/admin/faq", label: "FAQ", icon: HelpCircle },
   { href: "/admin/news", label: "Новости", icon: Newspaper },
+  { href: "/admin/popups", label: "Всплывающие окна", icon: MessageSquare },
   { href: "/admin/contact", label: "Контакты", icon: Phone },
 ];