/* ───────────────────────────────────────────
   Base
─────────────────────────────────────────── */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  scroll-behavior: smooth;
}

body {
  background: #1a1a1a;
  overflow-x: hidden;
}

/* ───────────────────────────────────────────
   Slides
─────────────────────────────────────────── */
main {
  display: block;
}

section {
  width: 100%;
  line-height: 0; /* remove gap under img */
}

section img {
  width: 100%;
  height: auto;
  display: block;
}

/* ───────────────────────────────────────────
   Side navigation
─────────────────────────────────────────── */
.nav {
  position: fixed;
  right: 1rem;
  top: 50%;
  transform: translateY(-50%);
  z-index: 100;
}

.nav ul {
  list-style: none;
  display: flex;
  flex-direction: column;
  gap: 3px;
}

.nav-item {
  display: flex;
  align-items: center;
  justify-content: center;
  width: 32px;
  height: 24px;
  border-radius: 4px;
  font-family: 'Roboto', 'Segoe UI', sans-serif;
  font-size: 11px;
  font-weight: 500;
  letter-spacing: 0.03em;
  color: rgba(255, 255, 255, 0.45);
  background: rgba(0, 0, 0, 0.25);
  text-decoration: none;
  transition: color 0.2s, background 0.2s;
  backdrop-filter: blur(4px);
  -webkit-backdrop-filter: blur(4px);
}

.nav-item:hover {
  color: #fff;
  background: rgba(14, 87, 164, 0.7);
}

.nav-item.active {
  color: #fff;
  background: #0e57a4;
}

/* ───────────────────────────────────────────
   Back to top button
─────────────────────────────────────────── */
.back-to-top {
  position: fixed;
  bottom: 1.5rem;
  right: 1rem;
  z-index: 100;
  width: 40px;
  height: 40px;
  border-radius: 8px;
  border: none;
  background: #0e57a4;
  color: #fff;
  font-size: 22px;
  line-height: 1;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  opacity: 0;
  pointer-events: none;
  transform: translateY(8px);
  transition: opacity 0.25s, transform 0.25s;
}

.back-to-top.visible {
  opacity: 1;
  pointer-events: auto;
  transform: translateY(0);
}

.back-to-top:hover {
  background: #2953a1;
}

/* ───────────────────────────────────────────
   Responsive — tablet / mobile
─────────────────────────────────────────── */

/* Hide nav numbers on narrow screens, show only dots */
@media (max-width: 768px) {
  .nav {
    right: 0.5rem;
  }

  .nav-item {
    width: 10px;
    height: 10px;
    border-radius: 50%;
    font-size: 0;       /* hide text, show only colored circle */
    padding: 0;
  }

  .back-to-top {
    bottom: 1rem;
    right: 0.5rem;
    width: 36px;
    height: 36px;
    font-size: 18px;
  }
}

@media (max-width: 480px) {
  .nav {
    right: 4px;
  }

  .nav-item {
    width: 8px;
    height: 8px;
  }
}