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/app/api/open-day-register/route.ts

@@ -58,6 +58,7 @@ export async function POST(request: NextRequest) {
     if (msg.includes("UNIQUE")) {
       return NextResponse.json({ error: "Вы уже записаны на это занятие" }, { status: 409 });
     }
-    return NextResponse.json({ error: msg }, { status: 500 });
+    console.error("[open-day-register] POST error:", e);
+    return NextResponse.json({ error: "Internal error" }, { status: 500 });
   }
 }