fix: security hardening, UI fixes, and validation improvements

- Fix header nav overflow by switching to lg: breakpoint with tighter gaps
- Fix file upload path traversal by whitelisting allowed folders and extensions
- Fix BookingModal using hardcoded content instead of DB-backed data
- Add input length validation on public master-class registration API
- Add ID validation on team member and reorder API routes
- Fix BookingModal useCallback missing groupInfo/contact dependencies
- Improve admin news date field to use native date picker
- Add missing Мастер-классы and Новости cards to admin dashboard

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

src/app/admin/page.tsx

@@ -5,9 +5,11 @@ import {
   FileText,
   Users,
   BookOpen,
+  Star,
   Calendar,
   DollarSign,
   HelpCircle,
+  Newspaper,
   Phone,
 } from "lucide-react";
 
@@ -17,9 +19,11 @@ const CARDS = [
   { href: "/admin/about", label: "О студии", icon: FileText, desc: "Текст о студии" },
   { href: "/admin/team", label: "Команда", icon: Users, desc: "Тренеры и инструкторы" },
   { href: "/admin/classes", label: "Направления", icon: BookOpen, desc: "Типы занятий" },
+  { href: "/admin/master-classes", label: "Мастер-классы", icon: Star, desc: "Мастер-классы и записи" },
   { href: "/admin/schedule", label: "Расписание", icon: Calendar, desc: "Расписание занятий" },
   { href: "/admin/pricing", label: "Цены", icon: DollarSign, desc: "Абонементы и аренда" },
   { href: "/admin/faq", label: "FAQ", icon: HelpCircle, desc: "Часто задаваемые вопросы" },
+  { href: "/admin/news", label: "Новости", icon: Newspaper, desc: "Новости и анонсы" },
   { href: "/admin/contact", label: "Контакты", icon: Phone, desc: "Адреса, телефон, карта" },
 ];