/* ============================================================
   Fundamentos de la Física — diseño "electric dark"
   Dark por defecto (modo claro disponible con el toggle ☀️/🌙).
   Estética: neón eléctrico (violeta/cian) sobre fondo profundo,
   pero con tipografía grande y alto contraste para leer en
   movimiento. Si tocás colores, mantené contraste AA en texto.
   ============================================================ */
:root {
  --bg: #f3f4fb;
  --bg-glow-1: rgba(124, 58, 237, .10);
  --bg-glow-2: rgba(34, 211, 238, .08);
  --surface: #ffffff;
  --surface-2: #e9ebf6;
  --text: #16182b;
  --text-soft: #5a6080;
  --primary: #6d28d9;
  --primary-strong: #7c3aed;
  --primary-soft: rgba(124, 58, 237, .10);
  --neon: #0891b2;
  --neon-soft: rgba(8, 145, 178, .10);
  --accent: #059669;
  --accent-soft: rgba(5, 150, 105, .10);
  --warn: #b45309;
  --warn-soft: rgba(180, 83, 9, .10);
  --error: #e11d48;
  --error-soft: rgba(225, 29, 72, .10);
  --border: rgba(124, 58, 237, .25);
  --border-soft: rgba(90, 96, 128, .22);
  --grad: linear-gradient(135deg, #8b5cf6 0%, #6366f1 45%, #22d3ee 100%);
  --glow-card: 0 1px 4px rgba(22, 24, 60, .10), 0 6px 20px rgba(22, 24, 60, .08);
  --glow-neon: 0 0 14px rgba(8, 145, 178, .25);
  --radius: 16px;
  --maxw: 880px;
  --mono: ui-monospace, "Cascadia Code", Consolas, "Roboto Mono", monospace;
}

html.dark {
  --bg: #07080f;
  --bg-glow-1: rgba(124, 58, 237, .16);
  --bg-glow-2: rgba(34, 211, 238, .10);
  --surface: #10121f;
  --surface-2: #1a1d31;
  --text: #eceefb;
  --text-soft: #9298bd;
  --primary: #a78bfa;
  --primary-strong: #8b5cf6;
  --primary-soft: rgba(139, 92, 246, .16);
  --neon: #22d3ee;
  --neon-soft: rgba(34, 211, 238, .12);
  --accent: #34d399;
  --accent-soft: rgba(52, 211, 153, .13);
  --warn: #fbbf24;
  --warn-soft: rgba(251, 191, 36, .13);
  --error: #fb7185;
  --error-soft: rgba(251, 113, 133, .14);
  --border: rgba(139, 92, 246, .22);
  --border-soft: rgba(146, 152, 189, .18);
  --glow-card: 0 0 0 1px var(--border), 0 8px 28px rgba(0, 0, 0, .45);
  --glow-neon: 0 0 18px rgba(34, 211, 238, .35);
}

* { box-sizing: border-box; }

html, body { margin: 0; padding: 0; }

body {
  font-family: system-ui, -apple-system, "Segoe UI", Roboto, sans-serif;
  background:
    radial-gradient(900px 500px at -10% -5%, var(--bg-glow-1), transparent 60%),
    radial-gradient(800px 520px at 110% 100%, var(--bg-glow-2), transparent 60%),
    var(--bg);
  background-attachment: fixed;
  color: var(--text);
  font-size: 17px;
  line-height: 1.65;
  -webkit-font-smoothing: antialiased;
}

/* Grilla técnica sutil de fondo */
body::before {
  content: "";
  position: fixed;
  inset: 0;
  z-index: -1;
  background-image:
    linear-gradient(var(--border-soft) 1px, transparent 1px),
    linear-gradient(90deg, var(--border-soft) 1px, transparent 1px);
  background-size: 44px 44px;
  opacity: .14;
  pointer-events: none;
}

/* ============ Barra superior ============ */
.topbar {
  position: sticky;
  top: 0;
  z-index: 20;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 8px;
  padding: 10px 16px;
  padding-top: calc(10px + env(safe-area-inset-top));
  background: color-mix(in srgb, var(--surface) 82%, transparent);
  backdrop-filter: blur(12px);
  -webkit-backdrop-filter: blur(12px);
  border-bottom: 1px solid var(--border);
}

.brand {
  display: flex;
  align-items: center;
  gap: 9px;
  text-decoration: none;
  color: var(--text);
  font-weight: 800;
  font-size: 1.02rem;
  letter-spacing: .04em;
  text-transform: uppercase;
}

.brand-icon { filter: drop-shadow(0 0 6px rgba(34, 211, 238, .8)); }

.icon-btn {
  border: 1px solid var(--border);
  background: var(--surface-2);
  border-radius: 10px;
  font-size: 1.1rem;
  padding: 6px 10px;
  cursor: pointer;
  color: var(--text);
  transition: box-shadow .15s ease;
}

.icon-btn:hover { box-shadow: var(--glow-neon); }

/* ============ Contenido ============ */
.content {
  max-width: var(--maxw);
  margin: 0 auto;
  padding: 18px 16px 96px;
  outline: none;
}

h1 {
  font-size: 1.65rem;
  line-height: 1.25;
  margin: 10px 0 6px;
  font-weight: 900;
  letter-spacing: -.01em;
  background: var(--grad);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
  width: fit-content;
}

h2 {
  font-size: 1.05rem;
  margin: 26px 0 10px;
  font-weight: 800;
  text-transform: uppercase;
  letter-spacing: .09em;
  color: var(--neon);
}

h2::before { content: "▸ "; color: var(--primary); }

h3 { font-size: 1.05rem; margin: 16px 0 6px; font-weight: 700; }
p { margin: 8px 0; }

.muted { color: var(--text-soft); font-size: .92rem; }

a { color: var(--neon); }

/* ============ Navegación inferior ============ */
.bottombar {
  position: fixed;
  bottom: 0;
  left: 0;
  right: 0;
  z-index: 20;
  display: flex;
  justify-content: space-around;
  background: color-mix(in srgb, var(--surface) 85%, transparent);
  backdrop-filter: blur(14px);
  -webkit-backdrop-filter: blur(14px);
  border-top: 1px solid var(--border);
  padding-bottom: env(safe-area-inset-bottom);
}

.bottombar a {
  flex: 1;
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 1px;
  padding: 9px 2px 7px;
  text-decoration: none;
  color: var(--text-soft);
  font-size: .68rem;
  font-family: var(--mono);
  letter-spacing: .05em;
  text-transform: uppercase;
  border-top: 2px solid transparent;
  transition: color .15s ease;
}

.bottombar a.active {
  color: var(--neon);
  border-top-color: var(--neon);
  text-shadow: 0 0 12px rgba(34, 211, 238, .6);
  font-weight: 700;
}

.nav-ico { font-size: 1.3rem; line-height: 1.2; }

/* ============ Tarjetas ============ */
.card {
  background: var(--surface);
  border: 1px solid var(--border-soft);
  border-radius: var(--radius);
  padding: 16px;
  margin: 12px 0;
  box-shadow: var(--glow-card);
  position: relative;
  overflow: hidden;
}

.card-link {
  display: block;
  text-decoration: none;
  color: var(--text);
  transition: transform .1s ease, border-color .15s ease, box-shadow .15s ease;
}

.card-link::after {
  content: "→";
  position: absolute;
  right: 14px;
  top: 14px;
  color: var(--primary);
  font-weight: 800;
  opacity: .55;
}

.card-link:hover, .card-link:focus-visible {
  border-color: var(--neon);
  box-shadow: var(--glow-card), var(--glow-neon);
}

.card-link:active { transform: scale(.985); }

.card-title {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 800;
  font-size: 1.05rem;
  padding-right: 22px;
}

.grid-2 {
  display: grid;
  grid-template-columns: 1fr;
  gap: 12px;
}

@media (min-width: 640px) {
  .grid-2 { grid-template-columns: 1fr 1fr; }
}

.chip {
  display: inline-block;
  background: var(--surface-2);
  border: 1px solid var(--border-soft);
  border-radius: 7px;
  padding: 2px 10px;
  font-size: .74rem;
  font-family: var(--mono);
  letter-spacing: .03em;
  color: var(--text-soft);
  margin: 2px 4px 2px 0;
}

.chip.ok {
  background: var(--accent-soft);
  border-color: var(--accent);
  color: var(--accent);
  font-weight: 700;
  text-shadow: 0 0 10px rgba(52, 211, 153, .5);
}

.pill-row { margin: 6px 0; }

/* ============ Botones ============ */
.btn {
  display: inline-block;
  border: none;
  border-radius: 12px;
  padding: 13px 20px;
  font-size: 1rem;
  font-weight: 800;
  letter-spacing: .02em;
  cursor: pointer;
  background: var(--grad);
  color: #fff;
  text-decoration: none;
  text-align: center;
  transition: filter .12s ease, transform .08s ease, box-shadow .15s ease;
  box-shadow: 0 4px 18px rgba(99, 102, 241, .35);
}

.btn:hover { filter: brightness(1.12); box-shadow: 0 4px 22px rgba(34, 211, 238, .45); }
.btn:active { transform: scale(.97); }

.btn.secondary {
  background: var(--surface-2);
  color: var(--text);
  border: 1px solid var(--border);
  box-shadow: none;
}

.btn.secondary:hover { border-color: var(--neon); box-shadow: var(--glow-neon); filter: none; }

.btn.success { background: linear-gradient(135deg, #059669, #34d399); box-shadow: 0 4px 18px rgba(52, 211, 153, .3); }
.btn.warn { background: linear-gradient(135deg, #d97706, #fbbf24); color: #1c1404; box-shadow: 0 4px 18px rgba(251, 191, 36, .25); }
.btn.small { padding: 8px 14px; font-size: .9rem; }
.btn.block { display: block; width: 100%; }
.btn[disabled] { opacity: .5; cursor: default; }

.btn-row { display: flex; gap: 10px; flex-wrap: wrap; margin: 14px 0; }
.btn-row .btn { flex: 1; min-width: 130px; }

/* ============ Fórmulas y contenido teórico ============ */
.formula {
  background: linear-gradient(180deg, var(--neon-soft), transparent 140%), var(--surface);
  border: 1px solid var(--border-soft);
  border-left: 3px solid var(--neon);
  border-radius: 12px;
  padding: 13px 15px;
  margin: 14px 0;
  font-family: Georgia, "Times New Roman", serif;
  font-size: 1.18rem;
  letter-spacing: .03em;
  overflow-x: auto;
  white-space: pre-line;
  box-shadow: inset 0 0 24px rgba(34, 211, 238, .05);
}

.formula .fdesc {
  display: block;
  font-family: system-ui, sans-serif;
  font-size: .83rem;
  color: var(--text-soft);
  letter-spacing: 0;
  margin-top: 7px;
}

.idea {
  background: linear-gradient(180deg, var(--accent-soft), transparent 160%), var(--surface);
  border: 1px solid var(--border-soft);
  border-left: 3px solid var(--accent);
  border-radius: 12px;
  padding: 12px 14px;
  margin: 14px 0;
}

.idea::before {
  content: "⚡ IDEA CLAVE";
  display: block;
  font-weight: 800;
  font-family: var(--mono);
  font-size: .76rem;
  letter-spacing: .12em;
  margin-bottom: 4px;
  color: var(--accent);
}

.ojo {
  background: linear-gradient(180deg, var(--warn-soft), transparent 160%), var(--surface);
  border: 1px solid var(--border-soft);
  border-left: 3px solid var(--warn);
  border-radius: 12px;
  padding: 12px 14px;
  margin: 14px 0;
}

.ojo::before {
  content: "⚠ OJO";
  display: block;
  font-weight: 800;
  font-family: var(--mono);
  font-size: .76rem;
  letter-spacing: .12em;
  margin-bottom: 4px;
  color: var(--warn);
}

.paso-calculo {
  background: var(--surface-2);
  border: 1px solid var(--border-soft);
  border-radius: 12px;
  padding: 13px 15px;
  margin: 10px 0;
  font-family: Georgia, serif;
  font-size: 1.06rem;
  line-height: 1.95;
  overflow-x: auto;
  white-space: pre-line;
}

table.tabla {
  width: 100%;
  border-collapse: collapse;
  margin: 12px 0;
  font-size: .92rem;
}

table.tabla th, table.tabla td {
  border: 1px solid var(--border-soft);
  padding: 8px 10px;
  text-align: left;
}

table.tabla th {
  background: var(--surface-2);
  font-family: var(--mono);
  font-size: .8rem;
  letter-spacing: .06em;
  text-transform: uppercase;
  color: var(--neon);
}

.lesson-body ul { padding-left: 22px; }
.lesson-body li { margin: 7px 0; }
.lesson-body strong { color: var(--primary); }

/* ============ Flashcards ============ */
.fcard {
  perspective: 1200px;
  margin: 16px 0;
  cursor: pointer;
  -webkit-tap-highlight-color: transparent;
}

.fcard-inner {
  position: relative;
  width: 100%;
  min-height: 260px;
  transition: transform .45s;
  transform-style: preserve-3d;
}

.fcard.flipped .fcard-inner { transform: rotateY(180deg); }

.fcard-face {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 10px;
  padding: 22px;
  border-radius: var(--radius);
  border: 1px solid var(--border);
  box-shadow: var(--glow-card);
  text-align: center;
}

.fcard-front {
  background:
    radial-gradient(420px 200px at 50% -20%, var(--primary-soft), transparent 70%),
    var(--surface);
}

.fcard-back {
  background:
    radial-gradient(420px 200px at 50% 120%, var(--neon-soft), transparent 70%),
    var(--surface-2);
  border-color: var(--neon);
  box-shadow: var(--glow-card), var(--glow-neon);
  transform: rotateY(180deg);
  font-size: 1.02rem;
}

.fcard-hint {
  font-size: .72rem;
  font-family: var(--mono);
  letter-spacing: .1em;
  text-transform: uppercase;
  color: var(--text-soft);
}

.fcard-q { font-size: 1.18rem; font-weight: 800; }

/* ============ Quiz ============ */
.quiz-opt {
  display: block;
  width: 100%;
  text-align: left;
  background: var(--surface-2);
  border: 1px solid var(--border-soft);
  border-radius: 12px;
  padding: 13px 15px;
  margin: 8px 0;
  font-size: 1rem;
  cursor: pointer;
  color: var(--text);
  line-height: 1.45;
  transition: border-color .12s ease, box-shadow .12s ease;
}

.quiz-opt:not([disabled]):hover { border-color: var(--neon); box-shadow: var(--glow-neon); }

.quiz-opt.correct {
  border-color: var(--accent);
  background: var(--accent-soft);
  font-weight: 700;
  box-shadow: 0 0 16px rgba(52, 211, 153, .3);
}

.quiz-opt.wrong {
  border-color: var(--error);
  background: var(--error-soft);
}

.quiz-expl {
  border-radius: 12px;
  padding: 12px 14px;
  margin: 12px 0;
  border: 1px solid var(--border-soft);
}

.quiz-expl.good { background: var(--accent-soft); border-left: 3px solid var(--accent); }
.quiz-expl.bad { background: var(--error-soft); border-left: 3px solid var(--error); }

.progress-track {
  height: 7px;
  background: var(--surface-2);
  border-radius: 99px;
  overflow: hidden;
  margin: 10px 0;
  border: 1px solid var(--border-soft);
}

.progress-fill {
  height: 100%;
  background: var(--grad);
  border-radius: 99px;
  transition: width .3s ease;
  box-shadow: 0 0 10px rgba(34, 211, 238, .6);
}

/* ============ Ejercicios guiados ============ */
.step {
  border: 1px solid var(--border-soft);
  border-radius: 12px;
  margin: 10px 0;
  overflow: hidden;
  background: var(--surface);
}

.step-q {
  padding: 12px 14px;
  font-weight: 700;
}

.step-q .step-num {
  display: inline-block;
  background: var(--grad);
  color: #fff;
  border-radius: 8px;
  min-width: 28px;
  text-align: center;
  font-family: var(--mono);
  font-size: .85rem;
  padding: 2px 6px;
  margin-right: 8px;
  box-shadow: 0 0 10px rgba(99, 102, 241, .5);
}

.step-body { padding: 0 14px 14px; display: none; }
.step.open { border-color: var(--border); }
.step.open .step-body { display: block; }

.datos {
  background: var(--surface-2);
  border: 1px solid var(--border-soft);
  border-radius: 12px;
  padding: 12px 14px;
  margin: 10px 0;
}

.datos strong {
  font-family: var(--mono);
  font-size: .8rem;
  letter-spacing: .1em;
  color: var(--neon);
}

.datos ul { margin: 4px 0; padding-left: 20px; }

.stars { color: var(--warn); letter-spacing: 2px; text-shadow: 0 0 8px rgba(251, 191, 36, .5); }

/* ============ Acordeón (finales) ============ */
.acc {
  border: 1px solid var(--border-soft);
  border-radius: 12px;
  margin: 10px 0;
  background: var(--surface);
  overflow: hidden;
  transition: border-color .15s ease;
}

.acc.open { border-color: var(--border); }

.acc-head {
  width: 100%;
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 10px;
  padding: 14px;
  background: none;
  border: none;
  font: inherit;
  font-weight: 700;
  text-align: left;
  cursor: pointer;
  color: var(--text);
}

.acc-head::after { content: "▾"; color: var(--neon); transition: transform .2s; }
.acc.open .acc-head::after { transform: rotate(180deg); }
.acc-body { display: none; padding: 0 14px 14px; }
.acc.open .acc-body { display: block; }

.puntos-clave {
  background: var(--primary-soft);
  border: 1px solid var(--border-soft);
  border-left: 3px solid var(--primary-strong);
  border-radius: 12px;
  padding: 10px 14px;
  margin: 10px 0;
}

.puntos-clave strong {
  font-family: var(--mono);
  font-size: .76rem;
  letter-spacing: .1em;
  color: var(--primary);
}

/* ============ Check de progreso manual ============ */
.check-btn {
  display: flex;
  align-items: center;
  gap: 10px;
  width: 100%;
  background: var(--surface-2);
  border: 1px dashed var(--border);
  border-radius: 12px;
  padding: 13px 14px;
  font: inherit;
  cursor: pointer;
  color: var(--text-soft);
  margin: 18px 0 6px;
  transition: border-color .15s ease, box-shadow .15s ease;
}

.check-btn:hover { border-color: var(--neon); }

.check-btn.done {
  background: var(--accent-soft);
  border: 1px solid var(--accent);
  color: var(--accent);
  font-weight: 700;
  box-shadow: 0 0 14px rgba(52, 211, 153, .25);
}

/* ============ Inicio ============ */
.hero {
  position: relative;
  background:
    radial-gradient(500px 240px at 85% -30%, rgba(34, 211, 238, .35), transparent 65%),
    radial-gradient(400px 240px at 0% 120%, rgba(139, 92, 246, .45), transparent 65%),
    linear-gradient(135deg, #1e1b4b 0%, #312e81 55%, #155e75 100%);
  color: #fff;
  border: 1px solid var(--border);
  border-radius: var(--radius);
  padding: 22px 18px;
  margin: 8px 0 16px;
  overflow: hidden;
  box-shadow: var(--glow-card);
}

.hero::after {
  content: "⚡";
  position: absolute;
  right: -6px;
  bottom: -22px;
  font-size: 7rem;
  opacity: .12;
  transform: rotate(12deg);
  pointer-events: none;
}

.hero h1 {
  margin: 0 0 4px;
  background: linear-gradient(90deg, #fff, #a5f3fc);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}

.hero p { color: #d6dcff; margin: 4px 0; font-size: .96rem; }
.hero strong { color: #67e8f9; }

.tiempo-row { display: grid; grid-template-columns: repeat(3, 1fr); gap: 10px; margin: 12px 0; }

.tiempo-card {
  background: var(--surface);
  border: 1px solid var(--border-soft);
  border-radius: 12px;
  padding: 13px 8px;
  text-align: center;
  text-decoration: none;
  color: var(--text);
  box-shadow: var(--glow-card);
  transition: border-color .15s ease, box-shadow .15s ease, transform .1s ease;
}

.tiempo-card:hover { border-color: var(--neon); box-shadow: var(--glow-card), var(--glow-neon); }
.tiempo-card:active { transform: scale(.97); }

.tiempo-card .t-min {
  font-weight: 800;
  font-family: var(--mono);
  color: var(--neon);
  font-size: 1.1rem;
  text-shadow: 0 0 12px rgba(34, 211, 238, .5);
}

.tiempo-card .t-what { font-size: .78rem; color: var(--text-soft); }

.breadcrumb {
  font-size: .8rem;
  font-family: var(--mono);
  letter-spacing: .04em;
  margin-bottom: 4px;
}

.breadcrumb a { text-decoration: none; }

.resultado-final {
  background: var(--accent-soft);
  border: 1px solid var(--accent);
  border-radius: 12px;
  padding: 14px;
  margin: 14px 0;
  font-weight: 700;
  box-shadow: 0 0 18px rgba(52, 211, 153, .2);
}

.score-big {
  font-size: 3rem;
  font-weight: 900;
  font-family: var(--mono);
  text-align: center;
  margin: 10px 0;
  background: var(--grad);
  -webkit-background-clip: text;
  background-clip: text;
  -webkit-text-fill-color: transparent;
}

@media (min-width: 700px) {
  body { font-size: 18px; }
  .content { padding-top: 28px; }
  h1 { font-size: 1.9rem; }
}

/* Respeto por usuarios con animaciones reducidas */
@media (prefers-reduced-motion: reduce) {
  .fcard-inner { transition: none; }
  .progress-fill { transition: none; }
}