/* Empira — Topic page v2 (rebuild Phase 1)
   Design system: Inter font, #3B4CFF blue, #F4F5F9 background.
   No gamification, no stars. */

:root {
  --blue-500: #3B4CFF;  --blue-600: #2F3DE0;  --blue-700: #2632C2;
  --blue-100: #D7DBFF;  --blue-50:  #EEF0FF;
  --amber-500: #FFB21E;
  --green-500: #1FA85C;  --green-50: #E7F7EE;
  --amber-50: #FFF3DC;
  --ink-900: #1A1D33;  --ink-700: #474C63;  --ink-500: #6B7191;  --ink-400: #8A90A6;  --ink-300: #9AA0B4;  --ink-200: #AEB4C6;  --ink-100: #C3C7D6;
  --line: #ECEDF3;  --line-dashed: #D4D8E6;
  --mist: #ECEEF4;  --paper: #F4F5F9;  --white: #FFFFFF;

  --font: 'Inter', system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;

  --s1: 4px; --s2: 8px; --s3: 12px; --s4: 16px; --s5: 20px; --s6: 24px;
  --s7: 32px; --s8: 48px;

  --r-s: 7px; --r-m: 12px; --r-l: 18px; --r-xl: 22px; --r-full: 999px;
  --ease: cubic-bezier(.2,.8,.2,1); --t-fast: 150ms;

  --page-gutter: 18px;
  --topbar-h: 62px;
}

*, *::before, *::after { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0; min-height: 100vh;
  background: var(--paper); color: var(--ink-900);
  font-family: var(--font); font-size: 16px; line-height: 1.55;
  -webkit-font-smoothing: antialiased;
}
h1, h2, h3, p { margin: 0; font: inherit; }
ol, ul { margin: 0; padding: 0; list-style: none; }
a { color: inherit; text-decoration: none; }
button { font: inherit; color: inherit; background: none; border: 0; padding: 0; cursor: pointer; }
[hidden] { display: none !important; }
:focus-visible { outline: 2px solid var(--blue-500); outline-offset: 2px; }

/* ========== Top bar ========== */
.topbar {
  position: sticky; top: 0; z-index: 10;
  background: var(--white);
  border-bottom: 1px solid var(--line);
}
.topbar__inner {
  max-width: 1300px;
  margin: 0 auto;
  padding: 0 var(--page-gutter);
  height: var(--topbar-h);
  display: flex; align-items: center; gap: 10px;
}
.topbar__back {
  width: 38px; height: 38px;
  border-radius: var(--r-m);
  background: var(--paper);
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0; color: var(--ink-900);
  transition: background var(--t-fast) var(--ease);
}
.topbar__class { flex: 1; min-width: 0; }
.topbar__class-name { display: block; font-size: 15px; font-weight: 700; color: var(--ink-900); }
.topbar__class-subject { display: block; font-size: 12px; font-weight: 500; color: var(--ink-400); }

/* Desktop-only elements hidden on phone */
.topbar__logo { display: none; }
.topbar__right { display: none; }

/* ========== Page content ========== */
.page {
  max-width: 1300px;
  margin: 0 auto;
  padding: var(--page-gutter);
  padding-bottom: calc(var(--s8) + env(safe-area-inset-bottom));
}

/* ========== Back link (desktop) ========== */
.back-link {
  display: none; /* shown on desktop */
}

/* ========== Chapter badge ========== */
.chapter-badge {
  display: inline-flex; align-items: center;
  background: var(--blue-50); border-radius: var(--r-full);
  padding: 6px 12px; margin-bottom: 12px;
}
.chapter-badge__text {
  font-size: 12px; font-weight: 700; color: var(--blue-500);
  letter-spacing: 0.01em;
}

/* ========== Topic title ========== */
.topic-title {
  margin: 0 0 10px;
  font-size: 27px; font-weight: 800; color: var(--ink-900);
  letter-spacing: -0.02em; line-height: 1.15;
}

/* ========== Main layout: info + actions ========== */
.topic-layout {
  display: flex; flex-direction: column; gap: 22px;
}

/* ========== Info card ========== */
.info-card {
  background: var(--white);
  border: 1px solid var(--line); border-radius: var(--r-l);
  padding: 17px;
}
.info-card__label {
  font-size: 12px; font-weight: 700; color: var(--ink-300);
  text-transform: uppercase; letter-spacing: 0.05em;
  margin-bottom: 9px;
}
.info-card__text {
  font-size: 15px; line-height: 1.5; color: var(--ink-700); font-weight: 500;
  margin-bottom: 13px;
}
.info-card__text:last-child { margin-bottom: 0; }

/* Concept items (I, II, III style) */
.concepts { display: flex; flex-direction: column; gap: 9px; }
.concept {
  display: flex; align-items: flex-start; gap: 9px;
}
.concept__num {
  width: 22px; height: 22px; border-radius: var(--r-s);
  background: var(--blue-50); color: var(--blue-500);
  font-size: 12px; font-weight: 800;
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0; margin-top: 1px;
}
.concept__text {
  font-size: 14px; color: var(--ink-700); font-weight: 500; line-height: 1.45;
}

/* Formula inside info card */
.info-card__formula {
  margin-top: 13px; padding-top: 13px;
  border-top: 1px solid var(--line);
}
.info-card__formula-expr {
  font-size: 22px; font-weight: 700; color: var(--ink-900);
  letter-spacing: -0.01em; font-variant-numeric: tabular-nums;
}

/* ========== Action cards section ========== */
.actions-label {
  font-size: 12px; font-weight: 700; color: var(--ink-300);
  text-transform: uppercase; letter-spacing: 0.05em;
  margin: 0 2px 11px;
}
.actions { display: flex; flex-direction: column; gap: 12px; }

/* Primary action (lab) */
.action-card {
  width: 100%; display: flex; align-items: center; gap: 14px;
  border: none; border-radius: var(--r-l);
  padding: 18px; cursor: pointer; text-align: left;
  transition: transform var(--t-fast) var(--ease), box-shadow var(--t-fast) var(--ease);
}
.action-card--primary {
  background: var(--blue-500);
  box-shadow: 0 8px 20px rgba(59, 76, 255, 0.26);
}
.action-card--secondary {
  background: var(--white);
  border: 1px solid var(--line);
}
.action-card--disabled {
  background: #F7F8FB;
  border: 1px dashed var(--line-dashed);
  cursor: not-allowed;
}
.action-card__icon {
  width: 46px; height: 46px; border-radius: 13px;
  display: flex; align-items: center; justify-content: center;
  flex-shrink: 0;
}
.action-card--primary .action-card__icon { background: rgba(255,255,255,0.18); }
.action-card__body { flex: 1; min-width: 0; }
.action-card__title {
  font-size: 17px; font-weight: 700;
}
.action-card--primary .action-card__title { color: var(--white); }
.action-card--secondary .action-card__title { color: var(--ink-900); }
.action-card--disabled .action-card__title { color: var(--ink-300); }
.action-card__subtitle {
  font-size: 13px; font-weight: 500; margin-top: 2px;
}
.action-card--primary .action-card__subtitle { color: rgba(255,255,255,0.82); }
.action-card--secondary .action-card__subtitle { color: var(--ink-400); }
.action-card--disabled .action-card__subtitle { color: var(--ink-200); }
.action-card__arrow { flex-shrink: 0; }
.action-card__badge {
  background: #FFF0D2; color: #C98400;
  font-size: 12px; font-weight: 700;
  border-radius: var(--r-full); padding: 5px 11px;
  flex-shrink: 0;
}

/* ========== Phone prev/next nav ========== */
.pager-phone {
  display: flex; gap: 10px; margin-top: 22px;
}
.pager-phone__tile {
  flex: 1; min-width: 0;
  display: flex; align-items: center; gap: 10px;
  background: var(--white);
  border: 1px solid var(--line); border-radius: var(--r-m);
  padding: 13px 14px;
  transition: background var(--t-fast) var(--ease), border-color var(--t-fast) var(--ease);
}
.pager-phone__tile--next {
  justify-content: flex-end; text-align: right;
  background: var(--blue-50);
  border-color: var(--blue-100);
}
.pager-phone__label {
  display: block; font-size: 11px; font-weight: 600; color: var(--ink-300);
}
.pager-phone__tile--next .pager-phone__label { color: #8A90FF; }
.pager-phone__title {
  display: block; font-size: 13px; font-weight: 700; color: var(--ink-900);
  white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
}
.pager-phone__tile--next .pager-phone__title { color: var(--blue-500); }

/* ========== Bottom nav (desktop only, sticky) ========== */
.bottom-nav {
  display: none; /* shown on desktop */
}

/* ========== "No lab" state ========== */
.nolab {
  background: var(--white);
  border: 1px solid var(--line); border-radius: var(--r-l);
  padding: 20px;
}
.nolab__text { font-size: 18px; font-weight: 600; color: var(--ink-900); }
.nolab .btn { margin-top: var(--s4); width: 100%; }

/* ========== Button ========== */
.btn {
  display: inline-flex; align-items: center; justify-content: center; gap: var(--s2);
  height: 48px; padding: 0 20px;
  border-radius: var(--r-m);
  font-size: 16px; font-weight: 600; line-height: 1; white-space: nowrap;
  transition: background var(--t-fast) var(--ease);
  -webkit-tap-highlight-color: transparent;
  border: none; cursor: pointer;
}
.btn--primary { background: var(--blue-500); color: var(--white); }
.btn--secondary { background: var(--white); color: var(--ink-900); border: 1px solid var(--line); }
.btn--large { height: 56px; padding: 0 var(--s6); }

/* ========== Interaction states ========== */
@media (hover: hover) {
  .topbar__back:hover { background: var(--blue-50); }
  .action-card--primary:hover { transform: translateY(-1px); box-shadow: 0 12px 28px rgba(59,76,255,0.32); }
  .action-card--secondary:hover { background: var(--blue-50); border-color: var(--blue-500); }
  .action-card--secondary:hover .action-card__title { color: var(--blue-500); }
  .pager-phone__tile:hover { background: var(--blue-50); border-color: var(--blue-500); }
  .pager-phone__tile--next:hover { background: var(--blue-50); border-color: var(--blue-500); }
  .btn--primary:hover { background: var(--blue-600); }
  .btn--secondary:hover { background: var(--blue-50); border-color: var(--blue-500); color: var(--blue-500); }
}
.action-card--primary:active { transform: scale(0.99); }
.action-card--secondary:active { background: var(--blue-50); border-color: var(--blue-600); }
.pager-phone__tile:active { background: var(--blue-50); }

/* ========== Tablet ≥ 640 ========== */
@media (min-width: 640px) {
  :root { --page-gutter: 24px; }
  .page { padding-top: 30px; }
  .topic-title { font-size: 32px; }
  .chapter-badge { padding: 7px 14px; }
  .chapter-badge__text { font-size: 14px; }
  .info-card { padding: 22px; }
  .info-card__text { font-size: 16px; }
  .concept__num { width: 28px; height: 28px; font-size: 13px; border-radius: 9px; }
  .concept__text { font-size: 15px; }
  .concepts { gap: 12px; }
  .action-card { padding: 20px; border-radius: var(--r-xl); }
  .action-card__icon { width: 52px; height: 52px; border-radius: 14px; }
  .action-card__title { font-size: 19px; }
  .action-card__subtitle { font-size: 14px; }
}

/* ========== Desktop ≥ 1024 ========== */
@media (min-width: 1024px) {
  :root { --page-gutter: 48px; --topbar-h: 68px; }

  /* Header: show logo + right side, hide phone class info */
  .topbar__class { display: none; }
  .topbar__logo {
    display: flex; align-items: center; gap: 11px;
    height: 44px; border-radius: var(--r-s);
  }
  .topbar__logo-text {
    font-size: 23px; font-weight: 800; color: var(--ink-900);
    letter-spacing: -0.02em;
  }
  .topbar__right {
    display: flex; align-items: center; gap: 14px;
    margin-left: auto;
  }
  .topbar__right-text {
    font-size: 16px; font-weight: 600; color: var(--ink-700);
  }
  .topbar__avatar {
    width: 44px; height: 44px; border-radius: 50%;
    background: #E7E9FF;
    display: flex; align-items: center; justify-content: center;
    font-weight: 700; color: var(--blue-500); font-size: 17px;
  }

  /* Back link inside page */
  .back-link {
    display: inline-flex; align-items: center; gap: 8px;
    color: var(--ink-500); font-size: 16px; font-weight: 600;
    margin-bottom: 18px;
    transition: color var(--t-fast) var(--ease);
  }
  .back-link:hover { color: var(--blue-500); }

  .page { padding: 30px 48px 48px; }
  .topic-title { font-size: 46px; letter-spacing: -0.025em; margin-bottom: 30px; }
  .chapter-badge { padding: 8px 16px; margin-bottom: 14px; }
  .chapter-badge__text { font-size: 15px; font-weight: 700; }

  /* Two-column layout */
  .topic-layout {
    flex-direction: row; flex-wrap: wrap;
    gap: 26px; align-items: flex-start;
  }
  .topic-layout__info {
    flex: 1 1 380px; min-width: 320px;
  }
  .topic-layout__actions {
    flex: 1.5 1 560px; min-width: 320px;
  }

  /* Info card desktop sizes */
  .info-card { padding: 30px; border-radius: 24px; }
  .info-card__label { margin-bottom: 14px; font-size: 14px; }
  .info-card__text { font-size: 18px; line-height: 1.55; margin-bottom: 22px; }
  .concept__num { width: 34px; height: 34px; font-size: 15px; border-radius: 10px; }
  .concept__text { font-size: 17px; padding-top: 4px; }
  .concepts { gap: 16px; }

  /* Actions: grid layout on desktop */
  .actions-label { font-size: 14px; margin-bottom: 16px; }
  .actions {
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(250px, 1fr));
    gap: 18px;
  }
  .action-card--primary {
    grid-column: 1 / -1;
    padding: 26px 28px;
    border-radius: var(--r-xl);
    box-shadow: 0 12px 28px rgba(59,76,255,0.26);
  }
  .action-card--primary .action-card__icon {
    width: 64px; height: 64px; border-radius: 17px;
  }
  .action-card--primary .action-card__title { font-size: 24px; }
  .action-card--primary .action-card__subtitle { font-size: 16px; margin-top: 4px; }
  .action-card--secondary {
    flex-direction: column; align-items: flex-start; gap: 16px;
    padding: 24px; border-radius: var(--r-xl);
    min-height: 150px;
  }
  .action-card--secondary .action-card__icon {
    width: 56px; height: 56px; border-radius: 15px;
  }
  .action-card--secondary .action-card__title { font-size: 21px; }
  .action-card--secondary .action-card__subtitle { font-size: 15px; margin-top: 4px; }
  .action-card--secondary .action-card__arrow { display: none; }
  .action-card--disabled {
    grid-column: 1 / -1;
    padding: 22px 26px; border-radius: var(--r-xl);
  }
  .action-card--disabled .action-card__icon {
    width: 56px; height: 56px; border-radius: 15px;
  }
  .action-card--disabled .action-card__title { font-size: 21px; }
  .action-card--disabled .action-card__subtitle { font-size: 15px; margin-top: 4px; }
  .action-card--disabled .action-card__badge { font-size: 14px; padding: 7px 15px; }

  .info-card__formula-expr { font-size: 28px; }

  /* Phone pager hidden on desktop */
  .pager-phone { display: none; }

  /* Bottom nav visible on desktop */
  .bottom-nav {
    display: block;
    position: sticky; bottom: 0; z-index: 10;
    background: var(--white);
    border-top: 1px solid var(--line);
    box-shadow: 0 -4px 20px rgba(20,22,40,0.06);
  }
  .bottom-nav__inner {
    max-width: 1300px;
    margin: 0 auto; padding: 16px 48px;
    display: flex; align-items: center; justify-content: space-between;
    gap: 16px;
  }
  .bottom-nav__tile {
    display: flex; align-items: center; gap: 14px;
    border-radius: var(--r-m);
    padding: 14px 22px;
    flex: 0 1 340px; min-width: 0;
    transition: background var(--t-fast) var(--ease), border-color var(--t-fast) var(--ease);
  }
  .bottom-nav__tile--prev {
    background: var(--paper); border-radius: 16px;
  }
  .bottom-nav__tile--next {
    background: var(--blue-500); border-radius: 16px;
    justify-content: flex-end; text-align: right;
  }
  .bottom-nav__tile-text { min-width: 0; }
  .bottom-nav__tile-label {
    display: block; font-size: 13px; font-weight: 600;
  }
  .bottom-nav__tile--prev .bottom-nav__tile-label { color: var(--ink-300); }
  .bottom-nav__tile--next .bottom-nav__tile-label { color: rgba(255,255,255,0.78); }
  .bottom-nav__tile-title {
    display: block; font-size: 17px; font-weight: 700;
    white-space: nowrap; overflow: hidden; text-overflow: ellipsis;
  }
  .bottom-nav__tile--prev .bottom-nav__tile-title { color: var(--ink-900); }
  .bottom-nav__tile--next .bottom-nav__tile-title { color: var(--white); }
  .bottom-nav__center {
    display: flex; flex-direction: column; align-items: center; gap: 2px;
    flex-shrink: 0; padding: 0 8px;
  }
  .bottom-nav__counter { font-size: 13px; color: var(--ink-300); font-weight: 600; }
  .bottom-nav__all { font-size: 15px; color: var(--blue-500); font-weight: 700; }
  .bottom-nav__tile[hidden] { visibility: hidden; }

  @media (hover: hover) {
    .bottom-nav__tile--prev:hover { background: var(--blue-50); }
    .bottom-nav__tile--next:hover { background: var(--blue-600); }
  }
}

@media (prefers-reduced-motion: reduce) {
  * { transition: none !important; }
}