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/MasterClasses.tsx

@@ -163,6 +163,7 @@ function MasterClassCard({
               onClick={() =>
                 window.open(item.instagramUrl, "_blank", "noopener,noreferrer")
               }
+              aria-label={`Instagram ${item.trainer}`}
               className="flex h-[46px] w-[46px] items-center justify-center rounded-xl border border-white/10 text-white/40 transition-all hover:border-gold/30 hover:text-gold cursor-pointer"
             >
               <Instagram size={18} />
@@ -225,9 +226,9 @@ export function MasterClasses({ data }: MasterClassesProps) {
         ) : (
           <Reveal>
             <div className="mx-auto mt-10 grid max-w-5xl grid-cols-1 gap-5 sm:grid-cols-2 lg:grid-cols-3">
-              {upcoming.map((item, i) => (
+              {upcoming.map((item) => (
                 <MasterClassCard
-                  key={i}
+                  key={item.title}
                   item={item}
                   onSignup={() => setSignupTitle(item.title)}
                 />