/* =========================
     Header
  ========================= */
  
  .header {
    position: sticky;
    top: 0;
    z-index: 100;
    background: var(--card);
    border-bottom: 1px solid var(--border);
    padding: 0.5rem 1rem 0.6rem;
    text-align: center;
  }
  
  /* ロゴ画像 */
    .logo a {
        display: inline-flex;
        align-items: center;
    }

    .logo-image {
        height: 80px;           /* 好みで調整（28〜40pxくらい） */
        margin-bottom: -7px;
        width: auto;
        display: block;
    }

    /* スマホでは少し小さく */
    @media (max-width: 600px) {
        .logo-image {
            height: 60px;
        }
    }
  
  .header-nav {
    margin-top: 0.35rem;
    display: flex;
    justify-content: center;
    gap: 1.8rem;
  }
  
  .nav-link {
    font-family: 'Poppins', sans-serif;
    font-size: 0.95rem;
    text-decoration: none;
    position: relative;
    color: #555;
    padding: 0.15rem 0;
    transition: color 0.3s ease;
  }
  
  .nav-link:hover {
    color: var(--accent);
  }

    .nav-link::after {
    content: '';
    position: absolute;
    bottom: 0;           /* 下線を文字の下に */
    left: 50%;
    transform: translateX(-50%) scaleX(0);
    width: 100%;
    height: 2px;
    background: var(--accent);
    transition: transform 0.3s ease;
    transform-origin: center;
    }

    .nav-link:hover::after {
    transform: translateX(-50%) scaleX(1);
    }

  .back-button {
    position: fixed;       /* 画面に固定 */
    top: 1rem;             /* 上から1rem */
    left: 1rem;            /* 左から1rem */
    z-index: 999;          /* navや記事より前面に表示 */
    padding: 0.5em 0.9em;
    font-size: 0.85rem;
    font-weight: 500;
    color: var(--accent);
    background: rgba(255,255,255,0.9);
    border-radius: 6px;
    text-decoration: none;
    box-shadow: 0 1px 4px rgba(0,0,0,0.15);
    transition: all 0.2s ease;
    }

    .back-button:hover {
    background: var(--accent);
    color: #fff;
    transform: translateY(-2px);
    box-shadow: 0 3px 6px rgba(0,0,0,0.2);
    }