/* RESET */
* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

html {
  height: 100%;
}

body {
  background: #050505;
  color: #f5f5f5;
  font-family: Arial, sans-serif;

  min-height: 100dvh; /* スマホ対応 */
  overflow-x: hidden;
  overflow-y: auto;
}

/* 背景 */
body::before {
  content: "";
  position: fixed;
  inset: 0;

  background:
    radial-gradient(
      circle at center,
      rgba(255, 255, 255, 0.28) 0%,
      rgba(255, 255, 255, 0.12) 16%,
      rgba(255, 255, 255, 0.03) 30%,
      transparent 48%
    ),
    radial-gradient(circle at center, rgba(255, 0, 0, 0.12), transparent 50%),
    linear-gradient(rgba(0, 0, 0, 0.25), rgba(0, 0, 0, 0.42)),
    url("../img/genkai.png");

  background-size: cover;
  background-position: center;

  transform: scale(1.03);
  animation: bgMove 8s ease-in-out infinite;

  z-index: -2;
}

.overlay {
  position: fixed;
  inset: 0;
  background: radial-gradient(
    circle,
    transparent 15%,
    rgba(0, 0, 0, 0.78) 100%
  );
  z-index: -1;
}

.hero {
  position: relative;
  width: 100%;
  min-height: 100dvh;

  display: flex;
  flex-direction: column;
  justify-content: center;
  align-items: center;

  text-align: center;
  padding: 20px;
}

/* ブランド */
.brand {
  position: absolute;
  top: 18%;
  width: 100%;
  text-align: center;

  font-size: 1.5rem;
  letter-spacing: 10px;

  opacity: 0.92;
}

/* ナンバー */
.number {
  font-size: 6rem;
}

/* タイトル */
h1 {
  font-size: 4rem;
}

/* サブ */
.sub {
  letter-spacing: 8px;
  margin-bottom: 40px;
  color: rgba(255, 255, 255, 0.75);
}

/* ボタン */
#playBtn {
  position: relative;
  z-index: 10;

  background: linear-gradient(
    135deg,
    rgba(90, 0, 0, 0.95),
    rgba(220, 0, 0, 0.95)
  );
  color: #fff;

  border: 1px solid rgba(255, 255, 255, 0.15);

  padding: 18px 54px;
  font-size: 1rem;
  font-weight: bold;

  letter-spacing: 4px;
  border-radius: 999px;

  cursor: pointer;

  box-shadow: 0 0 12px rgba(255, 0, 0, 0.25);
  transition: 0.3s ease;
}

/* hover（統一版） */
#playBtn:hover {
  transform: translateY(-3px) scale(1.05);
  box-shadow: 0 0 40px rgba(255, 0, 0, 0.65);
}

/* アニメ */
@keyframes pulse {
  0%,
  100% {
    transform: scale(1);
  }
  50% {
    transform: scale(1.08);
  }
}

@keyframes bgMove {
  0% {
    transform: scale(1.03);
    filter: brightness(1.05);
  }
  50% {
    transform: scale(1.08);
    filter: brightness(1.2);
  }
  100% {
    transform: scale(1.03);
    filter: brightness(0.95);
  }
}

/* スマホ対応 */
@media (max-width: 768px) {
  h1 {
    font-size: 2.2rem;
  }

  .number {
    font-size: 3.5rem;
  }

  .brand {
    font-size: 1rem;
    letter-spacing: 6px;
  }
}

/* ==========================

   HUB BACK BUTTON

========================== */

.hub-back {
  position: fixed;

  top: 20px;

  left: 20px;

  padding: 12px 20px;

  background: rgba(0, 0, 0, 0.55);

  border: 1px solid rgba(255, 255, 255, 0.12);

  border-radius: 999px;

  color: #fff;

  text-decoration: none;

  font-size: 0.9rem;

  letter-spacing: 1px;

  backdrop-filter: blur(12px);

  z-index: 9999;

  transition: all 0.3s ease;
}

.hub-back:hover {
  transform: translateY(-2px);

  border-color: rgba(255, 60, 60, 0.5);

  box-shadow: 0 0 25px rgba(255, 0, 0, 0.35);
}

/* スマホ */

@media (max-width: 768px) {
  .hub-back {
    top: 14px;

    left: 14px;

    font-size: 0.8rem;

    padding: 10px 16px;
  }
}