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/SignupModal.tsx

@@ -132,7 +132,7 @@ export function SignupModal({
   if (!open) return null;
 
   return createPortal(
-    <div className="modal-overlay fixed inset-0 z-50 flex items-center justify-center p-4" onClick={handleClose}>
+    <div className="modal-overlay fixed inset-0 z-50 flex items-center justify-center p-4" role="dialog" aria-modal="true" aria-label={title} onClick={handleClose}>
       <div className="absolute inset-0 bg-black/70 backdrop-blur-sm" />
       <div
         className="modal-content relative w-full max-w-md rounded-2xl border border-white/[0.08] bg-[#0a0a0a] p-6 sm:p-8 shadow-2xl"
@@ -140,6 +140,7 @@ export function SignupModal({
       >
         <button
           onClick={handleClose}
+          aria-label="Закрыть"
           className="absolute right-4 top-4 flex h-8 w-8 items-center justify-center rounded-full text-neutral-500 transition-colors hover:bg-white/[0.06] hover:text-white cursor-pointer"
         >
           <X size={18} />