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/ui/NewsModal.tsx

@@ -49,6 +49,9 @@ export function NewsModal({ item, onClose }: NewsModalProps) {
   return createPortal(
     <div
       className="modal-overlay fixed inset-0 z-50 flex items-center justify-center p-4"
+      role="dialog"
+      aria-modal="true"
+      aria-label={item.title}
       onClick={onClose}
     >
       <div className="absolute inset-0 bg-black/70 backdrop-blur-sm" />
@@ -59,6 +62,7 @@ export function NewsModal({ item, onClose }: NewsModalProps) {
       >
         <button
           onClick={onClose}
+          aria-label="Закрыть"
           className="absolute right-4 top-4 z-10 flex h-8 w-8 items-center justify-center rounded-full bg-black/50 text-neutral-400 backdrop-blur-sm transition-colors hover:bg-white/[0.1] hover:text-white cursor-pointer"
         >
           <X size={18} />