:root {
  --green: #2db84b;
  --green-dark: #239a3d;
  --blue: #2987cd;
  --ink: #1f2d3d;
  --muted: #6b7785;
  --line: #e4e8ec;
  --bg: #f5f7f9;
  --card: #ffffff;
  --code-bg: #1c2331;
  --code-ink: #e7edf5;
  --red: #d64545;
  --amber: #c98a00;
  --radius: 10px;
  --shadow: 0 1px 2px rgba(16, 24, 40, 0.06), 0 4px 14px rgba(16, 24, 40, 0.05);
}

* { box-sizing: border-box; }

html, body {
  margin: 0;
  padding: 0;
  background: var(--bg);
  color: var(--ink);
  font-family: "Lato", -apple-system, BlinkMacSystemFont, "Segoe UI", Helvetica, Arial, sans-serif;
  font-size: 15px;
  line-height: 1.5;
}

a { color: inherit; text-decoration: none; }
[hidden] { display: none !important; }

/* ---- Gate ---- */
.gate {
  position: fixed;
  inset: 0;
  display: grid;
  place-items: center;
  background: radial-gradient(1200px 600px at 50% -10%, #eaf8ee, var(--bg));
  padding: 20px;
}
.gate-card {
  width: 100%;
  max-width: 380px;
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: 16px;
  box-shadow: var(--shadow);
  padding: 34px 30px;
  text-align: center;
}
.gate-logo {
  font-size: 28px;
  font-weight: 800;
  letter-spacing: -0.02em;
  margin-bottom: 10px;
}
.logo-code { color: var(--green); }
.logo-world { color: var(--ink); }
.gate-card h1 { font-size: 20px; margin: 0 0 4px; }
.gate-sub { color: var(--muted); margin: 0 0 22px; font-size: 14px; }
.gate-card input {
  width: 100%;
  padding: 12px 14px;
  font: inherit;
  border: 1px solid var(--line);
  border-radius: 8px;
  margin-bottom: 12px;
  text-align: center;
  letter-spacing: 2px;
}
.gate-card input:focus { outline: none; border-color: var(--green); box-shadow: 0 0 0 3px rgba(45, 184, 75, 0.18); }
.gate-card button {
  width: 100%;
  padding: 12px;
  font: inherit;
  font-weight: 700;
  color: #fff;
  background: var(--green);
  border: none;
  border-radius: 8px;
  cursor: pointer;
}
.gate-card button:hover { background: var(--green-dark); }
.gate-error { color: var(--red); margin: 12px 0 0; font-size: 14px; }

/* ---- Nav ---- */
.nav {
  position: sticky;
  top: 0;
  z-index: 50;
  background: var(--green);
  box-shadow: 0 2px 10px rgba(0, 0, 0, 0.12);
}
.nav-inner {
  max-width: 900px;
  margin: 0 auto;
  height: 60px;
  padding: 0 20px;
  display: flex;
  align-items: center;
  gap: 16px;
}
.logo { display: inline-flex; align-items: baseline; font-size: 22px; font-weight: 800; letter-spacing: -0.02em; }
.nav .logo-code { color: #fff; }
.nav .logo-world { color: rgba(255, 255, 255, 0.72); }
.nav-tag { color: rgba(255, 255, 255, 0.85); font-size: 13px; font-weight: 600; }
.nav-right { margin-left: auto; display: flex; align-items: center; gap: 12px; }
.count { color: rgba(255, 255, 255, 0.9); font-size: 13px; font-weight: 600; }
.lock-btn {
  background: rgba(255, 255, 255, 0.16);
  border: 1px solid rgba(255, 255, 255, 0.28);
  color: #fff;
  font: inherit;
  font-size: 13px;
  font-weight: 600;
  padding: 6px 12px;
  border-radius: 8px;
  cursor: pointer;
}
.lock-btn:hover { background: rgba(255, 255, 255, 0.26); }

/* ---- App ---- */
#app { max-width: 900px; margin: 0 auto; padding: 24px 20px 80px; }

.hero {
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 22px 24px;
  margin-bottom: 20px;
}
.hero h1 { margin: 0 0 2px; font-size: 22px; }
.hero-sub { margin: 0 0 16px; color: var(--muted); }
.controls { display: flex; flex-wrap: wrap; gap: 12px; align-items: center; }
.search {
  flex: 1 1 220px;
  padding: 10px 14px;
  font: inherit;
  border: 1px solid var(--line);
  border-radius: 8px;
}
.search:focus { outline: none; border-color: var(--green); box-shadow: 0 0 0 3px rgba(45, 184, 75, 0.15); }
.chips { display: flex; gap: 8px; }
.chip {
  font: inherit;
  font-size: 13px;
  font-weight: 600;
  padding: 8px 14px;
  border-radius: 999px;
  border: 1px solid var(--line);
  background: #fff;
  color: var(--ink);
  cursor: pointer;
}
.chip:hover { border-color: var(--green); }
.chip.active { background: var(--green); border-color: var(--green); color: #fff; }

/* ---- List ---- */
.list { display: flex; flex-direction: column; gap: 10px; }
.challenge {
  background: var(--card);
  border: 1px solid var(--line);
  border-radius: var(--radius);
  box-shadow: var(--shadow);
  padding: 16px 20px;
  cursor: pointer;
  transition: border-color 0.12s ease, transform 0.12s ease;
}
.challenge:hover { border-color: var(--green); transform: translateY(-1px); }
.challenge-head { display: flex; align-items: center; gap: 12px; }
.challenge-title { font-size: 17px; margin: 0; flex: 1; }
.challenge-tags { margin-top: 8px; display: flex; flex-wrap: wrap; gap: 6px; }
.tag {
  font-size: 12px;
  font-weight: 600;
  color: var(--muted);
  background: var(--bg);
  border: 1px solid var(--line);
  border-radius: 6px;
  padding: 2px 8px;
}
.badge {
  font-size: 12px;
  font-weight: 700;
  padding: 3px 10px;
  border-radius: 999px;
  white-space: nowrap;
}
.badge.easy { background: #eaf8ee; color: var(--green-dark); }
.badge.medium { background: #fdf3e0; color: var(--amber); }
.empty { text-align: center; color: var(--muted); padding: 40px 0; }

/* ---- Detail ---- */
.detail { background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); box-shadow: var(--shadow); padding: 22px 26px; }
.back {
  font: inherit;
  font-size: 14px;
  font-weight: 600;
  color: var(--green-dark);
  background: none;
  border: none;
  cursor: pointer;
  padding: 0;
  margin-bottom: 12px;
}
.back:hover { text-decoration: underline; }
.detail-head { display: flex; align-items: center; gap: 12px; }
.detail-head h1 { margin: 0; font-size: 24px; flex: 1; }
.detail-tags { margin: 12px 0 18px; display: flex; flex-wrap: wrap; gap: 6px; }
.instructions h3, .detail h3 { margin: 22px 0 8px; font-size: 15px; }
.markdown { overflow-wrap: anywhere; }
.markdown h1, .markdown h2, .markdown h3 { font-size: 1.05em; }
.markdown pre { background: var(--code-bg); color: var(--code-ink); padding: 12px 14px; border-radius: 8px; overflow-x: auto; }
.markdown code { background: var(--bg); border: 1px solid var(--line); border-radius: 4px; padding: 1px 5px; font-size: 0.9em; }
.markdown pre code { background: none; border: none; padding: 0; color: inherit; }
.markdown blockquote { margin: 12px 0; padding: 2px 14px; border-left: 3px solid var(--green); color: var(--muted); }
.markdown img { max-width: 100%; }
.code {
  background: var(--code-bg);
  color: var(--code-ink);
  padding: 14px 16px;
  border-radius: 8px;
  overflow-x: auto;
  font-family: "SF Mono", "JetBrains Mono", Menlo, Consolas, monospace;
  font-size: 13px;
  line-height: 1.6;
  white-space: pre;
}
.src { margin-top: 18px; }
.src a { color: var(--blue); font-weight: 600; }

@media (max-width: 640px) {
  .nav-tag { display: none; }
  .controls { flex-direction: column; align-items: stretch; }
}