* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: "Segoe UI", Roboto, Arial, sans-serif;
  line-height: 1.6;
  color: black;
  background: #ffffff;
}

/* Nút hamburger (góc trên trái) */
#menu-btn {
  position: fixed;
  top: 16px;
  left: 16px;
  z-index: 20;
  font-size: 26px;
  line-height: 1;
  padding: 6px 12px;
  background: #ffffff;
  color: black;
  border: 1px solid #d4d4d4;
  border-radius: 8px;
  cursor: pointer;
}
#menu-btn:hover { background: #f0f0f0; }

/* Thanh menu bên trái */
#side-nav {
  position: fixed;
  top: 0;
  left: 0;
  width: 240px;
  height: 100%;
  padding: 80px 20px 20px;
  background: black;
  transform: translateX(-100%);
  transition: transform 0.3s ease;
  z-index: 10;
}
#side-nav a {
  display: block;
  padding: 12px 8px;
  color: #ffffff;
  text-decoration: none;
  font-size: 18px;
  border-bottom: 1px solid rgba(255, 255, 255, 0.15);
}
#side-nav a:hover { background: rgba(255, 255, 255, 0.12); }

/* Vùng nội dung: bị đẩy sang phải khi menu mở */
#page {
  transition: margin-left 0.3s ease;
}
body.nav-open #side-nav { transform: translateX(0); }
body.nav-open #page { margin-left: 240px; }

/* Trang chính khi mới vào web */
.hero {
  min-height: 35vh;
  display: flex;
  justify-content: center;
  align-items: center;
  text-align: center;
  padding: 20px;
  background: transparent;
}
.hero h1 {
  margin: 0;
  font-size: clamp(2rem, 6vw, 4rem);
  color: black;
}

/* Các section nội dung */
section {
  max-width: 900px;
  margin: 0 auto;
  padding: 70px 24px;
  scroll-margin-top: 20px;
}
#about { padding-top: 20px; }

section h2 {
  margin-top: 0;
  color: black;
  border-bottom: 2px solid #e5e5e5;
  padding-bottom: 8px;
}

/* Mobile: menu đè lên nội dung, không đẩy */
@media (max-width: 700px) {
  body.nav-open #page { margin-left: 0; }
  section { padding: 50px 16px; }
}