:root {
  --ink: #1a1f2e;
  --ink-soft: #5b6478;
  --paper: #f3f5fb;
  --card: #ffffff;
  --line: #dde2f0;
  --blue: #3454d1;
  --blue-dark: #24399a;
  --violet: #7b4fd6;
  --amber: #c07a17;
  --error: #a32d2d;
  --proton: #d1495b;
  --neutron: #4b5262;
  --electron: #2f9e6f;
  --radius: 12px;
}
* { box-sizing: border-box; }
body {
  margin: 0;
  font-family: "Be Vietnam Pro", system-ui, sans-serif;
  background: var(--paper);
  color: var(--ink);
}
.topbar {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 16px 32px;
  background: var(--card);
  border-bottom: 1px solid var(--line);
  flex-wrap: wrap;
  gap: 16px;
}
.brand { display: flex; align-items: center; gap: 12px; }
.brand-mark { font-size: 26px; color: var(--blue); }
.brand-name { font-weight: 700; font-size: 17px; }
.brand-sub { font-size: 12.5px; color: var(--ink-soft); }
.tabs { display: flex; gap: 4px; flex-wrap: wrap; }
.tab-btn {
  border: none; background: transparent; padding: 9px 16px; border-radius: 999px;
  font-size: 14px; color: var(--ink-soft); cursor: pointer; transition: all 0.15s;
  font-family: inherit;
}
.tab-btn:hover { background: #eef0fa; }
.tab-btn.active { background: var(--blue); color: white; }

main { max-width: 920px; margin: 0 auto; padding: 40px 24px 90px; }
.panel { display: none; }
.panel.active { display: block; }
h1 { font-size: 26px; font-weight: 700; margin: 0 0 6px; }
.sub { color: var(--ink-soft); margin: 0 0 26px; font-size: 15px; }
code { font-family: "IBM Plex Mono", monospace; background: #eef0fa; padding: 2px 6px; border-radius: 4px; font-size: 13px; }

/* --- Controls --- */
.control-block { margin-bottom: 22px; }
.control-label { font-size: 13px; color: var(--ink-soft); margin-bottom: 8px; font-weight: 600; }
.z-row { margin-bottom: 8px; }
.z-badge {
  display: inline-block; font-family: "IBM Plex Mono", monospace; font-size: 17px;
  font-weight: 700; background: var(--blue); color: white; padding: 5px 16px; border-radius: 999px;
}
input[type="range"] { width: 100%; accent-color: var(--blue); }
select, input[type="number"], input[type="text"] {
  font-family: "IBM Plex Mono", monospace; font-size: 14px; padding: 9px 12px;
  border: 1px solid var(--line); border-radius: 8px; background: var(--card); color: var(--ink);
}
.form-row { display: flex; align-items: end; gap: 16px; flex-wrap: wrap; margin-bottom: 22px; }
.form-row label { display: flex; flex-direction: column; gap: 6px; font-size: 13px; color: var(--ink-soft); }

/* --- Atom tab layout --- */
.atom-summary {
  position: sticky; top: 10px; z-index: 5;
  display: flex; align-items: center; gap: 14px; flex-wrap: wrap;
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 14px 20px; margin-bottom: 24px;
  box-shadow: 0 4px 16px rgba(26, 31, 46, 0.06);
}
.atom-summary .as-name { font-size: 16.5px; font-weight: 700; }
.atom-summary .as-badge {
  font-family: "IBM Plex Mono", monospace; font-size: 12.5px; font-weight: 700;
  background: #eef0fa; color: var(--ink-soft); padding: 4px 11px; border-radius: 999px;
}
.atom-summary .as-config {
  font-family: "IBM Plex Mono", monospace; font-size: 15px; font-weight: 700;
  color: var(--blue-dark); margin-left: auto;
}
@media (max-width: 640px) { .atom-summary .as-config { margin-left: 0; } }

.atom-layout { display: grid; grid-template-columns: 300px 1fr; gap: 32px; align-items: start; }
@media (max-width: 720px) { .atom-layout { grid-template-columns: 1fr; } }
.particle-cards { display: flex; flex-direction: column; gap: 10px; margin-top: 8px; }
.particle-card {
  display: flex; align-items: center; justify-content: space-between;
  background: var(--card); border: 1px solid var(--line); border-radius: 10px;
  padding: 10px 14px; font-size: 14px;
}
.particle-dot { width: 12px; height: 12px; border-radius: 50%; display: inline-block; margin-right: 8px; }
.particle-card .count { font-family: "IBM Plex Mono", monospace; font-weight: 700; font-size: 16px; }
.atom-visual { display: flex; justify-content: center; align-items: center; }
#atom-svg { width: 100%; max-width: 400px; height: auto; }

.step-panel {
  margin-top: 24px; background: var(--card); border: 1px solid var(--line);
  border-radius: var(--radius); padding: 16px 20px;
}
.step-panel .step-line { font-size: 14px; line-height: 1.8; color: var(--ink-soft); padding-left: 14px; border-left: 2px solid var(--line); margin-bottom: 6px; }
.step-panel .step-line:last-child { margin-bottom: 0; color: var(--ink); font-weight: 600; border-left-color: var(--blue); }

/* --- Electron config tab --- */
.orbital-diagram { display: flex; flex-direction: column; gap: 14px; margin-bottom: 20px; }
.orbital-row { display: flex; align-items: center; gap: 12px; flex-wrap: wrap; }
.orbital-row-label { font-family: "IBM Plex Mono", monospace; font-weight: 700; width: 46px; color: var(--blue-dark); }
.orbital-box-group { display: flex; gap: 4px; }
.orbital-box {
  width: 34px; height: 34px; border: 2px solid var(--line); border-radius: 6px;
  display: flex; align-items: center; justify-content: center; gap: 1px;
  background: var(--card); font-size: 15px; font-weight: 700; color: var(--blue-dark);
}
.orbital-box.filled { border-color: var(--blue); background: #eaf0ff; }
.config-line {
  font-family: "IBM Plex Mono", monospace; font-size: 19px; font-weight: 700;
  color: var(--blue-dark); margin-bottom: 20px; padding: 14px 18px;
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
}
.shell-bars { display: flex; flex-direction: column; gap: 8px; margin-bottom: 20px; }
.shell-bar-row { display: flex; align-items: center; gap: 10px; font-size: 13px; }
.shell-bar-label { width: 90px; color: var(--ink-soft); font-family: "IBM Plex Mono", monospace; }
.shell-bar-track { flex: 1; background: #eef0fa; border-radius: 6px; height: 20px; overflow: hidden; }
.shell-bar-fill { height: 100%; background: linear-gradient(90deg, var(--blue), var(--violet)); border-radius: 6px; transition: width 0.2s; }
.shell-bar-count { width: 36px; text-align: right; font-family: "IBM Plex Mono", monospace; font-weight: 700; }

/* --- Isotope tab --- */
.iso-inputs { display: flex; flex-direction: column; gap: 18px; margin-bottom: 22px; }
.iso-card {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 16px 20px;
}
.iso-card-header { display: flex; justify-content: space-between; align-items: center; margin-bottom: 10px; }
.iso-card-title { font-family: "IBM Plex Mono", monospace; font-weight: 700; font-size: 15px; }
.iso-card-percent { font-family: "IBM Plex Mono", monospace; font-weight: 700; color: var(--blue); }
.iso-chart { display: flex; gap: 14px; align-items: end; height: 160px; margin-bottom: 20px; padding: 0 10px; }
.iso-bar-wrap { flex: 1; display: flex; flex-direction: column; align-items: center; justify-content: end; height: 100%; gap: 6px; }
.iso-bar { width: 60%; background: linear-gradient(180deg, var(--violet), var(--blue)); border-radius: 8px 8px 0 0; transition: height 0.2s; min-height: 4px; }
.iso-bar-label { font-size: 12.5px; color: var(--ink-soft); font-family: "IBM Plex Mono", monospace; }

.result-box {
  margin-top: 10px; padding: 16px 20px; border-radius: var(--radius);
  background: var(--card); border: 1px solid var(--line);
}
.result-box.error { border-color: #e57373; color: #c62828; background: #ffebee; }
.result-box.hidden { display: none; }
.result-headline {
  font-family: "IBM Plex Mono", monospace; font-size: 20px; font-weight: 700;
  color: var(--blue-dark); margin-bottom: 10px;
}

/* --- N-Z chart tab --- */
.checkbox-label { flex-direction: row !important; align-items: center; gap: 8px !important; font-size: 14px !important; color: var(--ink) !important; }
.checkbox-label input { accent-color: var(--blue); width: 16px; height: 16px; }
.nz-chart-wrap {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 16px; margin-bottom: 10px;
}
#nz-svg { width: 100%; height: auto; }
.nz-note { font-size: 12.5px; color: var(--ink-soft); margin: 0 0 22px; font-style: italic; }
.nz-table-wrap { overflow-x: auto; }
.nz-table { width: 100%; border-collapse: collapse; font-size: 13px; background: var(--card); border: 1px solid var(--line); border-radius: var(--radius); overflow: hidden; }
.nz-table th, .nz-table td { padding: 8px 12px; text-align: right; border-bottom: 1px solid var(--line); font-family: "IBM Plex Mono", monospace; }
.nz-table th:first-child, .nz-table td:first-child { text-align: left; font-family: "Be Vietnam Pro", sans-serif; }
.nz-table th { background: #f7f8fd; color: var(--ink-soft); font-weight: 600; font-family: "Be Vietnam Pro", sans-serif; font-size: 12.5px; position: sticky; top: 0; }
.nz-table tr:last-child td { border-bottom: none; }
.nz-table tr:hover td { background: #f7f8fd; }
.nz-point { cursor: pointer; }
.nz-point:hover circle { stroke: var(--ink); stroke-width: 2; }

/* --- Quiz bank tab --- */
.quiz-tabs { display: flex; gap: 4px; margin-bottom: 22px; border-bottom: 1px solid var(--line); }
.quiz-subtab-btn {
  border: none; background: transparent; padding: 10px 18px; font-size: 14.5px;
  color: var(--ink-soft); cursor: pointer; font-family: inherit; border-bottom: 2.5px solid transparent;
  margin-bottom: -1px;
}
.quiz-subtab-btn.active { color: var(--blue); border-bottom-color: var(--blue); font-weight: 600; }
.quiz-subpanel { display: none; }
.quiz-subpanel.active { display: block; }

.quiz-setup {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 20px 22px; margin-bottom: 22px;
}
.quiz-start-btn, .quiz-submit-btn, .quiz-retry-btn {
  font-family: inherit; font-size: 14.5px; font-weight: 600; border: none; border-radius: 999px;
  padding: 11px 26px; cursor: pointer; background: var(--blue); color: white; transition: opacity 0.15s;
}
.quiz-start-btn:hover, .quiz-submit-btn:hover, .quiz-retry-btn:hover { opacity: 0.88; }
.quiz-retry-btn { background: #eef0fa; color: var(--ink); }

.quiz-summary {
  position: sticky; top: 10px; z-index: 5;
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 16px 22px; margin-bottom: 22px; display: flex; align-items: center; gap: 20px; flex-wrap: wrap;
  box-shadow: 0 4px 16px rgba(26, 31, 46, 0.06);
}
.quiz-score-badge { font-family: "IBM Plex Mono", monospace; font-size: 22px; font-weight: 700; color: var(--blue-dark); }
.quiz-score-label { font-size: 13px; color: var(--ink-soft); }

.quiz-questions { display: flex; flex-direction: column; gap: 16px; }
.quiz-q-card {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 18px 22px;
}
.quiz-q-card.correct { border-color: #4caf7d; background: #f3faf6; }
.quiz-q-card.incorrect { border-color: #e57373; background: #fef5f5; }
.quiz-q-head { display: flex; gap: 10px; align-items: baseline; margin-bottom: 10px; }
.quiz-q-number { font-family: "IBM Plex Mono", monospace; font-weight: 700; color: var(--blue); flex-shrink: 0; }
.quiz-q-text { font-size: 15px; line-height: 1.6; }
.quiz-options { display: flex; flex-direction: column; gap: 8px; margin-left: 30px; }
.quiz-option {
  display: flex; align-items: flex-start; gap: 10px; padding: 8px 12px; border-radius: 8px;
  cursor: pointer; font-size: 14.5px; border: 1px solid transparent; transition: background 0.1s;
}
.quiz-option:hover { background: #f7f8fd; }
.quiz-option input { margin-top: 3px; accent-color: var(--blue); }
.quiz-option.opt-correct { background: #e8f7ef; border-color: #4caf7d; }
.quiz-option.opt-wrong-picked { background: #fdeaea; border-color: #e57373; }
.quiz-explain {
  margin: 14px 0 0 30px; padding: 10px 14px; background: #f7f8fd; border-radius: 8px;
  font-size: 13.5px; color: var(--ink-soft); border-left: 3px solid var(--blue);
}
.quiz-actions { display: flex; gap: 12px; margin-top: 24px; }

.essay-card {
  background: var(--card); border: 1px solid var(--line); border-radius: var(--radius);
  padding: 20px 22px; margin-bottom: 18px;
}
.essay-title { font-size: 16.5px; font-weight: 700; margin-bottom: 10px; }
.essay-prompt { font-size: 14.5px; line-height: 1.7; margin-bottom: 12px; }
.essay-toggle-btn {
  font-family: inherit; font-size: 13.5px; border: 1px solid var(--line); background: #eef0fa;
  color: var(--blue-dark); border-radius: 999px; padding: 7px 16px; cursor: pointer; font-weight: 600;
}
.essay-solution { margin-top: 14px; padding: 14px 16px; background: #f7f8fd; border-radius: 8px; font-size: 14px; line-height: 1.8; display: none; }
.essay-solution.shown { display: block; }

/* Khối khoá "bài tập mở rộng" cho user FREE/khách vãng lai (xem includes/tools/_essay_gate.php) */
.vip-lock-card {
  background: linear-gradient(180deg, #fff, #f7f8fd);
  border: 1px solid var(--line); border-radius: var(--radius);
  padding: 32px 28px; text-align: center; max-width: 560px; margin: 0 auto;
}
.vip-lock-icon { font-size: 34px; margin-bottom: 6px; }
.vip-lock-card h3 { margin: 0 0 10px; font-size: 19px; color: var(--ink); }
.vip-lock-card p { color: var(--ink-soft); font-size: 14.5px; line-height: 1.7; margin: 0 0 16px; }
.vip-lock-titles {
  list-style: none; margin: 0 0 22px; padding: 0; text-align: left;
  max-height: 220px; overflow-y: auto; border: 1px solid var(--line); border-radius: 10px;
  background: var(--card);
}
.vip-lock-titles li {
  padding: 10px 16px; font-size: 13.5px; color: var(--ink-soft);
  border-bottom: 1px solid var(--line); filter: blur(0.35px);
}
.vip-lock-titles li:last-child { border-bottom: none; }
.vip-lock-titles li::before { content: "🔒 "; opacity: .6; }
.btn-upgrade {
  display: inline-block; font-weight: 700; font-size: 14.5px; text-decoration: none;
  background: linear-gradient(90deg, var(--amber), #e0a53a); color: #fff;
  padding: 12px 28px; border-radius: 999px; box-shadow: 0 4px 14px rgba(192,122,23,.28);
}
.btn-upgrade:hover { opacity: .92; }
.vip-lock-note { margin-top: 12px !important; font-size: 13px !important; }
.vip-lock-note a { color: var(--blue); font-weight: 600; }
.tab-btn .vip-badge { font-size: 10.5px; margin-left: 3px; opacity: .85; }