fix: HIGH priority — scroll debounce, timing-safe auth, a11y, error logging, cleanup dead modals

- Header: throttle scroll handler via requestAnimationFrame (was firing 60+/sec)
- Auth: use crypto.timingSafeEqual for password and token signature comparison
- A11y: add role="dialog", aria-modal, aria-label to all modals (SignupModal, NewsModal, TeamProfile lightbox)
- A11y: add aria-label to close buttons, menu toggle (with aria-expanded), floating CTA
- A11y: add aria-label to MC Instagram buttons
- Error logging: add console.error with route names to all API catch blocks (admin + public)
- Fix open-day-register error leak (was returning raw DB error to client)
- Fix MasterClasses key={index} → key={item.title}
- Delete 3 unused modal components (BookingModal, MasterClassSignupModal, OpenDaySignupModal) — replaced by unified SignupModal

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

src/components/sections/team/TeamProfile.tsx

@@ -309,10 +309,14 @@ export function TeamProfile({ member, onBack, schedule }: TeamProfileProps) {
       {lightbox && (
         <div
           className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm p-4"
+          role="dialog"
+          aria-modal="true"
+          aria-label="Просмотр изображения"
           onClick={() => setLightbox(null)}
         >
           <button
             onClick={() => setLightbox(null)}
+            aria-label="Закрыть"
             className="absolute top-4 right-4 rounded-full bg-white/10 p-2 text-white hover:bg-white/20 transition-colors"
           >
             <X size={20} />