*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --color-primary:      #6366f1;
  --color-primary-dark: #4f46e5;
  --color-primary-glow: rgba(99,102,241,.25);
  --color-success:      #22c55e;
  --color-warning:      #f59e0b;
  --color-danger:       #ef4444;
  --color-bg:           #f1f5f9;
  --color-surface:      #ffffff;
  --color-text:         #0f172a;
  --color-text-muted:   #64748b;
  --color-border:       #e2e8f0;
  --radius:             16px;
  --radius-sm:          10px;
  --radius-xs:          6px;
  --shadow:             0 1px 3px rgba(0,0,0,.07), 0 1px 2px rgba(0,0,0,.05);
  --shadow-md:          0 4px 16px rgba(0,0,0,.08), 0 2px 6px rgba(0,0,0,.04);
  --shadow-lg:          0 12px 40px rgba(0,0,0,.12), 0 4px 16px rgba(0,0,0,.06);
  --transition:         .18s ease;
}

html, body {
  height: 100%;
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  background: var(--color-bg);
  color: var(--color-text);
  font-size: 16px;
  line-height: 1.5;
  -webkit-font-smoothing: antialiased;
}

button   { cursor: pointer; border: none; font-family: inherit; font-size: inherit; }
a        { color: inherit; text-decoration: none; }
input, select, textarea { font-family: inherit; font-size: inherit; }

#content {
  min-height: 100vh;
  padding-bottom: calc(72px + env(safe-area-inset-bottom));
}

/* ---- Bottom nav ---- */
#nav {
  position: fixed;
  bottom: 0; left: 0; right: 0;
  background: rgba(255,255,255,.88);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border-top: 1px solid rgba(226,232,240,.8);
  display: flex;
  justify-content: space-around;
  padding: 8px 0 calc(8px + env(safe-area-inset-bottom));
  z-index: 100;
}
#nav a {
  display: flex;
  flex-direction: column;
  align-items: center;
  padding: 4px 12px;
  border-radius: var(--radius-sm);
  font-size: 12px;
  font-weight: 500;
  color: var(--color-text-muted);
  transition: color var(--transition);
}
#nav a.active { color: var(--color-primary); }

/* ---- Shared ---- */
.page    { padding: 16px; max-width: 700px; margin: 0 auto; }
.card    {
  background: var(--color-surface);
  border-radius: var(--radius);
  border: 1px solid var(--color-border);
  padding: 16px;
  margin-bottom: 12px;
  box-shadow: var(--shadow);
  transition: box-shadow var(--transition);
}
.centered { display: flex; align-items: center; justify-content: center; height: 60vh; color: var(--color-text-muted); }

.btn-primary {
  width: 100%;
  padding: 14px;
  background: linear-gradient(135deg, var(--color-primary), var(--color-primary-dark));
  color: #fff;
  border-radius: var(--radius-sm);
  font-size: 16px;
  font-weight: 600;
  cursor: pointer;
  border: none;
  box-shadow: 0 4px 14px var(--color-primary-glow);
  transition: transform var(--transition), box-shadow var(--transition), opacity var(--transition);
}
.btn-primary:active {
  transform: scale(.98);
  box-shadow: 0 2px 8px var(--color-primary-glow);
}

.btn-outline {
  padding: 8px 16px;
  border-radius: var(--radius-sm);
  border: 1px solid var(--color-border);
  background: transparent;
  cursor: pointer;
  font-weight: 500;
  transition: background var(--transition), border-color var(--transition);
}
.btn-outline:hover { background: var(--color-bg); }

.error-msg   { background: #fef2f2; color: var(--color-danger); padding: 10px 14px; border-radius: var(--radius-sm); margin-bottom: 12px; font-size: 14px; }
.success-msg { background: #f0fdf4; color: #16a34a;             padding: 10px 14px; border-radius: var(--radius-sm); margin-bottom: 12px; font-size: 14px; }

/* ---- Login page ---- */
.login-page {
  display: flex;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  padding: 20px;
  background: linear-gradient(160deg, #eef2ff 0%, #f1f5f9 50%, #faf5ff 100%);
}
.login-box {
  text-align: center;
  max-width: 360px;
  width: 100%;
}
.login-logo {
  width: 72px; height: 72px;
  border-radius: 20px;
  background: linear-gradient(135deg, var(--color-primary), var(--color-primary-dark));
  display: flex; align-items: center; justify-content: center;
  font-size: 36px;
  margin: 0 auto 20px;
  box-shadow: 0 8px 24px var(--color-primary-glow);
}
.login-title { font-size: 34px; font-weight: 800; margin-bottom: 8px; letter-spacing: -.5px; }
.login-sub   { color: var(--color-text-muted); margin-bottom: 40px; font-size: 15px; }
.btn-google  {
  display: flex; align-items: center; justify-content: center; gap: 12px;
  width: 100%; padding: 14px 24px;
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  border-radius: var(--radius-sm);
  font-weight: 600; font-size: 16px;
  box-shadow: var(--shadow-md);
  cursor: pointer;
  color: var(--color-text);
  text-decoration: none;
  transition: transform var(--transition), box-shadow var(--transition);
}
.btn-google:hover  { transform: translateY(-1px); box-shadow: var(--shadow-lg); }
.btn-google:active { transform: scale(.98); }

/* ---- Flip card ---- */
.flip-card {
  perspective: 1200px;
  cursor: pointer;
  user-select: none;
  margin-bottom: 16px;
  -webkit-tap-highlight-color: transparent;
}
.flip-card-inner {
  position: relative;
  transform-style: preserve-3d;
  transition: transform .55s cubic-bezier(.4,0,.2,1);
  min-height: 270px;
  border-radius: var(--radius);
}
.flip-card-inner.flipped { transform: rotateY(180deg); }

.flip-card-front,
.flip-card-back {
  position: absolute;
  inset: 0;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  border-radius: var(--radius);
  background: var(--color-surface);
  border: 1px solid var(--color-border);
  box-shadow: var(--shadow-md);
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  padding: 28px 24px;
  overflow: hidden;
}
.flip-card-back {
  transform: rotateY(180deg);
  align-items: flex-start;
  justify-content: flex-start;
  overflow-y: auto;
}

/* Subtle shimmer on front when not yet flipped */
.flip-card-front::after {
  content: '';
  position: absolute;
  inset: 0;
  background: linear-gradient(135deg, rgba(99,102,241,.04) 0%, transparent 60%);
  border-radius: var(--radius);
  pointer-events: none;
}

.card-hint {
  font-size: 11px;
  font-weight: 600;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: var(--color-text-muted);
  margin-bottom: 16px;
}
.card-word {
  font-size: 32px;
  font-weight: 800;
  text-align: center;
  line-height: 1.2;
  color: var(--color-text);
  letter-spacing: -.3px;
  margin-bottom: 20px;
}
.card-tap {
  font-size: 13px;
  color: var(--color-border);
  font-weight: 500;
}

/* Back side content */
.card-definition {
  font-size: 18px;
  font-weight: 600;
  line-height: 1.45;
  color: var(--color-text);
  margin-bottom: 14px;
  width: 100%;
}
.btn-reveal-translation {
  width: 100%;
  padding: 10px;
  border: 1.5px dashed var(--color-border);
  border-radius: var(--radius-sm);
  background: transparent;
  color: var(--color-text-muted);
  font-size: 14px;
  font-weight: 500;
  cursor: pointer;
  margin: 4px 0 2px;
  transition: border-color var(--transition), color var(--transition);
}
.btn-reveal-translation:hover {
  border-color: var(--color-primary);
  color: var(--color-primary);
}

.card-translation {
  font-size: 15px;
  font-weight: 700;
  color: var(--color-primary);
  padding: 8px 14px;
  background: rgba(99,102,241,.07);
  border-radius: var(--radius-sm);
  margin-bottom: 14px;
  width: 100%;
  transition: opacity .35s ease, transform .35s ease;
}
.card-translation--hidden {
  opacity: 0;
  transform: translateY(6px);
  pointer-events: none;
}
.card-examples {
  width: 100%;
  display: flex;
  flex-direction: column;
  gap: 6px;
  margin-bottom: 12px;
}
.card-example {
  font-size: 13px;
  color: var(--color-text-muted);
  font-style: italic;
  line-height: 1.5;
}
.card-user-sentence {
  font-size: 13px;
  color: var(--color-primary);
  font-style: italic;
  border-left: 3px solid var(--color-primary);
  padding-left: 10px;
  line-height: 1.5;
  width: 100%;
  margin-top: 4px;
}

/* ---- Study progress ---- */
.study-progress {
  height: 5px;
  background: var(--color-border);
  border-radius: 3px;
  margin-bottom: 10px;
  overflow: hidden;
}
.study-progress-bar {
  height: 100%;
  background: linear-gradient(90deg, var(--color-primary), var(--color-primary-dark));
  border-radius: 3px;
  transition: width .4s ease;
}
.study-counter {
  text-align: center;
  color: var(--color-text-muted);
  font-size: 13px;
  font-weight: 500;
  margin-bottom: 16px;
}

/* ---- Response buttons ---- */
.resp-row {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  gap: 10px;
  transition: opacity .25s ease, transform .25s ease;
}
.resp-row--hidden {
  opacity: 0;
  pointer-events: none;
  transform: translateY(8px);
}
.resp-btn {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 5px;
  padding: 14px 6px;
  border-radius: var(--radius-sm);
  border: 2px solid;
  background: transparent;
  cursor: pointer;
  font-size: 13px;
  font-weight: 600;
  transition: transform var(--transition), background var(--transition), box-shadow var(--transition);
}
.resp-btn:active { transform: scale(.95); }

.resp-btn--bad    { color: var(--color-danger);  border-color: var(--color-danger); }
.resp-btn--unsure { color: var(--color-warning); border-color: var(--color-warning); }
.resp-btn--good   { color: var(--color-success); border-color: var(--color-success); }

.resp-btn--bad:hover    { background: rgba(239,68,68,.07); }
.resp-btn--unsure:hover { background: rgba(245,158,11,.07); }
.resp-btn--good:hover   { background: rgba(34,197,94,.07); }

.resp-icon { font-size: 22px; line-height: 1; }