:root {
  --sky: #38bdf8;
  --sky-deep: #0284c7;
  --sky-soft: #e0f2fe;
  --sky-line: #bae6fd;
  --ink: #1e293b;
  --ink-soft: #475569;
  --muted: #94a3b8;
  --bg: #ffffff;
  --sidebar-w: 280px;
}

* { box-sizing: border-box; }

html { scroll-behavior: smooth; }

body {
  margin: 0;
  font-family: "Inter", "Noto Sans JP", -apple-system, sans-serif;
  font-size: 18px;
  line-height: 1.75;
  color: var(--ink);
  background: var(--bg);
}

.layout {
  display: flex;
  min-height: 100vh;
}

/* ---------- Sidebar ---------- */
.sidebar {
  width: var(--sidebar-w);
  flex-shrink: 0;
  border-right: 1px solid var(--sky-line);
  background: linear-gradient(180deg, var(--sky-soft) 0%, #ffffff 60%);
}

.sidebar-inner {
  position: sticky;
  top: 0;
  padding: 3rem 2rem;
}

.brand {
  display: block;
  font-size: 1.6rem;
  font-weight: 700;
  color: var(--ink);
  text-decoration: none;
  letter-spacing: -0.02em;
  line-height: 1.25;
}
.brand:hover { color: var(--sky-deep); }

.brand-sub {
  margin: 0.6rem 0 2.5rem;
  font-size: 0.92rem;
  color: var(--ink-soft);
  line-height: 1.5;
}

.nav {
  display: flex;
  flex-direction: column;
  gap: 0.25rem;
}

.nav a {
  position: relative;
  padding: 0.55rem 0.9rem;
  font-size: 1.05rem;
  font-weight: 500;
  color: var(--ink-soft);
  text-decoration: none;
  border-radius: 8px;
  transition: background 0.15s, color 0.15s;
}
.nav a:hover {
  background: var(--sky-soft);
  color: var(--sky-deep);
}
.nav a.active {
  background: var(--sky);
  color: #ffffff;
  font-weight: 600;
}

.lang-switch {
  display: inline-block;
  margin-top: 2.5rem;
  padding: 0.4rem 1rem;
  font-size: 0.92rem;
  font-weight: 500;
  color: var(--sky-deep);
  text-decoration: none;
  border: 1px solid var(--sky-line);
  border-radius: 999px;
  transition: background 0.15s, color 0.15s;
}
.lang-switch:hover {
  background: var(--sky-deep);
  color: #ffffff;
  border-color: var(--sky-deep);
}

/* ---------- Content ---------- */
.content {
  flex: 1;
  display: flex;
  justify-content: center;
}

.content-inner {
  width: 100%;
  max-width: 760px;
  padding: 3.5rem 3rem 6rem;
}

.content-inner h1 {
  font-size: 2.6rem;
  font-weight: 700;
  letter-spacing: -0.03em;
  margin: 0 0 1.8rem;
  color: var(--ink);
}

.content-inner h2 {
  font-size: 1.55rem;
  font-weight: 700;
  margin: 2.8rem 0 1rem;
  padding-bottom: 0.4rem;
  border-bottom: 2px solid var(--sky-line);
  color: var(--ink);
}

.content-inner h3 {
  font-size: 1.2rem;
  font-weight: 600;
  margin: 1.8rem 0 0.6rem;
  color: var(--ink);
}

.content-inner p { margin: 0.9rem 0; }

.content-inner a {
  color: var(--sky-deep);
  text-decoration: none;
  border-bottom: 1px solid var(--sky-line);
  transition: border-color 0.15s;
}
.content-inner a:hover { border-bottom-color: var(--sky-deep); }

.content-inner ul {
  padding-left: 1.4rem;
  margin: 0.8rem 0;
}
.content-inner li { margin: 0.35rem 0; }
.content-inner ul ul { margin: 0.3rem 0; }

.content-inner hr {
  border: none;
  border-top: 1px solid var(--sky-line);
  margin: 2.2rem 0;
}

.content-inner strong { color: var(--ink); font-weight: 600; }

/* ---------- Hamburger button ---------- */
.menu-toggle {
  display: none;
  position: fixed;
  top: 1rem;
  right: 1rem;
  z-index: 300;
  width: 2.2rem;
  height: 2.2rem;
  background: rgba(255, 255, 255, 0.9);
  color: var(--ink-soft);
  border: 1px solid var(--sky-line);
  border-radius: 6px;
  font-size: 1.1rem;
  line-height: 1;
  cursor: pointer;
  align-items: center;
  justify-content: center;
  backdrop-filter: blur(4px);
}

/* ---------- Overlay ---------- */
.overlay {
  display: none;
  position: fixed;
  inset: 0;
  background: rgba(0, 0, 0, 0.35);
  z-index: 200;
}
.overlay.open { display: block; }

/* ---------- Responsive ---------- */
@media (max-width: 820px) {
  .menu-toggle { display: flex; }

  .sidebar {
    position: fixed;
    top: 0;
    left: calc(-1 * var(--sidebar-w) - 2px);
    height: 100vh;
    z-index: 250;
    border-right: 1px solid var(--sky-line);
    transition: left 0.25s ease;
    overflow-y: auto;
  }
  .sidebar.open { left: 0; }

  .sidebar-inner { position: static; padding: 2rem 1.5rem; }

  .content-inner { padding: 3rem 1.5rem 4rem; }
  .content-inner h1 { font-size: 2.1rem; }
  body { font-size: 17px; }
}