:root {
  --background: #f8f9ff;
  --surface: #ffffff;
  --text-primary: #1f1f24;
  --text-secondary: #4f5361;
  --accent: #4a6bf2;
  --shadow: 0 12px 24px rgba(74, 107, 242, 0.08);
  --radius: 14px;
  --transition: 0.25s ease;
}

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

body {
  font-family: 'Inter', system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
  background: var(--background);
  color: var(--text-primary);
  min-height: 100vh;
  display: flex;
  flex-direction: column;
}

main {
  flex: 1;
  width: min(1100px, 92vw);
  margin: 96px auto 96px;
}

.app-header {
  position: sticky;
  top: 0;
  z-index: 10;
  background: rgba(248, 249, 255, 0.92);
  backdrop-filter: blur(16px);
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 18px clamp(16px, 6vw, 48px);
  border-bottom: 1px solid rgba(74, 107, 242, 0.1);
}

.brand {
  font-weight: 600;
  letter-spacing: 0.02em;
  font-size: 1.15rem;
  color: var(--accent);
}

.tab-nav {
  display: flex;
  gap: 12px;
}

.tab-button {
  border: none;
  background: none;
  font: inherit;
  padding: 10px 18px;
  border-radius: 999px;
  color: var(--text-secondary);
  cursor: pointer;
  transition: background var(--transition), color var(--transition), transform var(--transition);
}

.tab-button:hover,
.tab-button:focus-visible {
  background: rgba(74, 107, 242, 0.08);
  color: var(--accent);
}

.tab-button.active {
  background: var(--accent);
  color: #fff;
  box-shadow: var(--shadow);
}

.tab-panel {
  display: none;
  animation: fadeIn 0.35s ease;
}

.tab-panel.active {
  display: block;
}

.tab-title {
  font-size: clamp(1.6rem, 3vw, 2.2rem);
  margin-bottom: 10px;
}

.tab-subtitle {
  color: var(--text-secondary);
  margin-bottom: 26px;
  max-width: 640px;
}

.card-grid {
  display: grid;
  gap: 18px;
  grid-template-columns: repeat(auto-fit, minmax(220px, 1fr));
}

.empty-state {
  padding: 32px;
  background: var(--surface);
  border-radius: var(--radius);
  box-shadow: 0 8px 22px rgba(15, 23, 42, 0.08);
  text-align: center;
  color: var(--text-secondary);
}

.card {
  background: var(--surface);
  border-radius: var(--radius);
  padding: 22px;
  box-shadow: 0 6px 18px rgba(15, 23, 42, 0.08);
  transition: transform var(--transition), box-shadow var(--transition);
  display: flex;
  flex-direction: column;
  gap: 12px;
}

.card:hover,
.card:focus-within {
  transform: translateY(-4px);
  box-shadow: 0 12px 32px rgba(15, 23, 42, 0.12);
}

.card-letter {
  font-size: 2.4rem;
  font-weight: 600;
  color: var(--accent);
}

.letter-pair {
  display: flex;
  gap: 12px;
  align-items: baseline;
  flex-wrap: wrap;
  line-height: 1;
}

.card-letter.letter-pair {
  justify-content: flex-start;
}

.letter-main {
  font-size: 1em;
}

.letter-secondary {
  font-size: 0.58em;
  font-weight: 500;
  color: var(--accent);
  letter-spacing: 0.04em;
}

.letter-separator {
  font-size: 0.65em;
  font-weight: 600;
  color: var(--accent);
  letter-spacing: 0.04em;
}

.card-meta {
  display: flex;
  flex-direction: column;
  gap: 6px;
  color: var(--text-secondary);
}

.status-chip {
  align-self: flex-start;
  font-size: 0.78rem;
  padding: 6px 12px;
  border-radius: 999px;
  background: rgba(74, 107, 242, 0.1);
  color: var(--accent);
  font-weight: 600;
  letter-spacing: 0.04em;
}

.status-new {
  background: rgba(74, 107, 242, 0.1);
  color: var(--accent);
}

.status-familiar {
  background: rgba(72, 199, 142, 0.15);
  color: #2d8f65;
}

.status-hard {
  background: rgba(255, 107, 107, 0.15);
  color: #d64545;
}

.status-button {
  align-self: flex-start;
  padding: 8px 14px;
  border-radius: 999px;
  border: 1px solid rgba(74, 107, 242, 0.25);
  background: transparent;
  color: var(--accent);
  font-weight: 500;
  cursor: pointer;
  transition: background var(--transition), color var(--transition), border var(--transition);
}

.status-button:hover,
.status-button:focus-visible {
  background: var(--accent);
  color: #fff;
  border-color: transparent;
}

.filter-bar {
  display: flex;
  flex-wrap: wrap;
  gap: 16px;
  padding: 18px;
  background: rgba(255, 255, 255, 0.7);
  border-radius: var(--radius);
  margin-bottom: 22px;
  box-shadow: 0 4px 16px rgba(15, 23, 42, 0.06);
}

.filter-group {
  display: flex;
  align-items: center;
  gap: 12px;
  flex-wrap: wrap;
}

.filter-label,
.control-label {
  font-weight: 600;
  color: var(--text-secondary);
  margin-right: 6px;
}

.filter-group label {
  display: flex;
  align-items: center;
  gap: 6px;
  font-size: 0.95rem;
  color: var(--text-secondary);
}

.practice-panel {
  position: relative;
  padding: 32px clamp(18px, 4vw, 42px);
  border-radius: calc(var(--radius) + 18px);
  background: linear-gradient(135deg, rgba(74, 107, 242, 0.04), rgba(255, 255, 255, 0.95));
  box-shadow: 0 45px 80px rgba(15, 23, 42, 0.15);
  overflow: hidden;
  isolation: isolate;
}

.practice-panel::before,
.practice-panel::after {
  content: '';
  position: absolute;
  width: clamp(180px, 28vw, 320px);
  height: clamp(180px, 28vw, 320px);
  border-radius: 50%;
  background: rgba(74, 107, 242, 0.15);
  filter: blur(50px);
  opacity: 0.5;
  z-index: 0;
}

.practice-panel::before {
  top: -100px;
  right: -40px;
}

.practice-panel::after {
  bottom: -120px;
  left: -60px;
  background: rgba(72, 199, 142, 0.15);
}

.practice-panel > * {
  position: relative;
  z-index: 1;
}

.direction-controls {
  display: flex;
  align-items: center;
  gap: clamp(12px, 3vw, 24px);
  margin-bottom: 28px;
  padding: 8px 0 18px;
  border-bottom: 1px solid rgba(74, 107, 242, 0.12);
}

.direction-controls .control-label {
  text-transform: uppercase;
  letter-spacing: 0.08em;
}

.direction-controls .segmented {
  background: transparent;
  border: none;
  box-shadow: none;
  padding: 0;
}

.direction-controls .segment-button {
  border-radius: calc(var(--radius) + 4px);
  border: 1px solid rgba(74, 107, 242, 0.18);
}

.segmented {
  display: inline-flex;
  background: rgba(74, 107, 242, 0.08);
  border-radius: 999px;
  padding: 4px;
  gap: 6px;
  flex-wrap: wrap;
  border: 1px solid rgba(74, 107, 242, 0.15);
  box-shadow: inset 0 1px 0 rgba(255, 255, 255, 0.6);
  backdrop-filter: blur(10px);
}

.segment-button {
  border: none;
  background: transparent;
  padding: 10px 18px;
  border-radius: 999px;
  color: var(--text-secondary);
  font-weight: 500;
  cursor: pointer;
  letter-spacing: 0.01em;
  transition: background var(--transition), color var(--transition), box-shadow var(--transition), transform var(--transition);
}

.segment-button.active {
  background: linear-gradient(135deg, #4a6bf2, #7c8ffb);
  color: #fff;
  box-shadow: 0 12px 20px rgba(74, 107, 242, 0.25);
  transform: translateY(-1px);
}

.segment-button:not(.active):hover,
.segment-button:not(.active):focus-visible {
  background: rgba(74, 107, 242, 0.12);
  color: var(--accent);
}

.flashcard-wrapper {
  display: grid;
  grid-template-columns: auto 1fr auto;
  align-items: center;
  gap: 22px;
  margin-bottom: 32px;
  padding: clamp(16px, 3vw, 26px);
  border-radius: calc(var(--radius) + 10px);
  background: rgba(255, 255, 255, 0.85);
  border: 1px solid rgba(74, 107, 242, 0.1);
  box-shadow: 0 35px 65px rgba(15, 23, 42, 0.18);
  backdrop-filter: blur(18px);
}

.nav-arrow {
  width: 54px;
  height: 54px;
  border-radius: 16px;
  border: 1px solid rgba(74, 107, 242, 0.2);
  background: rgba(255, 255, 255, 0.9);
  box-shadow: 0 18px 38px rgba(15, 23, 42, 0.18), inset 0 1px 0 rgba(255, 255, 255, 0.8);
  color: var(--accent);
  font-size: 1.35rem;
  line-height: 1;
  cursor: pointer;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  transition: transform var(--transition), box-shadow var(--transition), color var(--transition);
}

.nav-arrow:hover,
.nav-arrow:focus-visible {
  transform: translateY(-2px) scale(1.03);
  box-shadow: 0 22px 44px rgba(15, 23, 42, 0.2), inset 0 1px 0 rgba(255, 255, 255, 0.9);
}

.flashcard {
  background: rgba(255, 255, 255, 0.92);
  border-radius: calc(var(--radius) + 6px);
  height: clamp(220px, 40vh, 320px);
  display: flex;
  align-items: center;
  justify-content: center;
  perspective: 1200px;
  border: 1px solid rgba(74, 107, 242, 0.1);
  box-shadow: 0 26px 60px rgba(15, 23, 42, 0.16);
  position: relative;
  overflow: hidden;
  transition: box-shadow var(--transition), transform var(--transition);
  will-change: transform;
}

.flashcard:hover,
.flashcard:focus-within {
  box-shadow: 0 36px 70px rgba(15, 23, 42, 0.22);
  transform: translateY(-4px);
}

.flashcard::before {
  content: '';
  position: absolute;
  inset: 24px;
  border-radius: inherit;
  background: radial-gradient(circle at top left, rgba(74, 107, 242, 0.25), transparent 55%);
  filter: blur(6px);
  opacity: 0.7;
  z-index: 0;
}

.flashcard::after {
  content: attr(data-status);
  position: absolute;
  top: 18px;
  left: 24px;
  font-size: 0.8rem;
  letter-spacing: 0.05em;
  text-transform: uppercase;
  font-weight: 600;
  padding: 6px 12px;
  border-radius: 999px;
  background: rgba(74, 107, 242, 0.12);
  color: var(--accent);
  pointer-events: none;
  z-index: 2;
}

.flashcard[data-status='Familiar']::after {
  background: rgba(72, 199, 142, 0.18);
  color: #2d8f65;
}

.flashcard[data-status='Hard']::after {
  background: rgba(255, 107, 107, 0.18);
  color: #d64545;
}

.flashcard-inner {
  position: relative;
  width: 100%;
  height: 100%;
  border-radius: inherit;
  z-index: 1;
  transform-style: preserve-3d;
  transition: transform 0.6s ease;
}

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

.flashcard-face {
  position: absolute;
  inset: 0;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 12px;
  backface-visibility: hidden;
  border-radius: inherit;
  padding: 28px;
  background: rgba(255, 255, 255, 0.8);
  border: 1px solid rgba(74, 107, 242, 0.08);
  box-shadow: inset 0 10px 30px rgba(15, 23, 42, 0.04);
  backdrop-filter: blur(14px);
  text-align: center;
}

.flashcard-face.back {
  transform: rotateY(180deg);
  background: linear-gradient(135deg, rgba(74, 107, 242, 0.18), rgba(74, 107, 242, 0.02));
}

.flashcard-letter {
  font-size: clamp(2.6rem, 7vw, 4.8rem);
  font-weight: 700;
  margin-bottom: 12px;
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 20px;
  background: linear-gradient(120deg, #4a6bf2, #8ba8ff);
  background-clip: text;
  -webkit-background-clip: text;
  color: transparent;
  -webkit-text-fill-color: transparent;
  letter-spacing: 0.04em;
  text-shadow: 0 12px 32px rgba(74, 107, 242, 0.3);
  transition: transform var(--transition);
}

.flashcard:hover .flashcard-letter,
.flashcard:focus-within .flashcard-letter {
  transform: translateY(-2px);
}

.flashcard-hint {
  font-size: clamp(1.1rem, 3vw, 1.4rem);
  color: rgba(31, 31, 36, 0.78);
  text-align: center;
  max-width: 80%;
  line-height: 1.5;
  font-weight: 500;
  letter-spacing: 0.01em;
}

.quiz-panel {
  display: flex;
  flex-direction: column;
  gap: 16px;
  background: rgba(255, 255, 255, 0.92);
  border-radius: calc(var(--radius) + 8px);
  padding: 24px;
  border: 1px solid rgba(74, 107, 242, 0.12);
  box-shadow: 0 32px 60px rgba(15, 23, 42, 0.16);
  backdrop-filter: blur(14px);
}

.quiz-label {
  font-weight: 600;
  color: var(--text-secondary);
  text-transform: uppercase;
  letter-spacing: 0.1em;
  font-size: 0.85rem;
}

#quizInput {
  border: 1px solid rgba(74, 107, 242, 0.3);
  border-radius: 12px;
  padding: 12px 16px;
  font-size: 1.05rem;
  outline: none;
  transition: border var(--transition), box-shadow var(--transition), background var(--transition);
  background: rgba(248, 249, 255, 0.9);
  font-weight: 500;
}

#quizInput:focus {
  border-color: var(--accent);
  box-shadow: 0 0 0 4px rgba(74, 107, 242, 0.12);
  background: #fff;
}

#quizInput::placeholder {
  color: rgba(79, 83, 97, 0.7);
}

.quiz-actions {
  display: flex;
  gap: 12px;
  flex-wrap: wrap;
}

.primary,
.ghost {
  border-radius: 999px;
  padding: 10px 20px;
  font-weight: 600;
  cursor: pointer;
  border: none;
  transition: transform var(--transition), box-shadow var(--transition), background var(--transition);
}

.primary {
  background: var(--accent);
  color: #fff;
  box-shadow: 0 12px 28px rgba(74, 107, 242, 0.18);
}

.primary:hover,
.primary:focus-visible {
  transform: translateY(-2px);
  box-shadow: 0 16px 32px rgba(74, 107, 242, 0.24);
}

.ghost {
  background: rgba(74, 107, 242, 0.12);
  color: var(--accent);
}

.quiz-feedback {
  min-height: 24px;
  font-weight: 500;
  padding-top: 4px;
  font-size: 0.95rem;
}

.quiz-feedback.success {
  color: #2d8f65;
}

.quiz-feedback.error {
  color: #d64545;
}

.mobile-nav {
  display: none;
}

@media (max-width: 980px) {
  main {
    width: min(720px, 94vw);
  }
}

@media (max-width: 720px) {
  body {
    padding-bottom: 76px;
  }

  .app-header {
    position: fixed;
    width: 100%;
    top: 0;
    left: 0;
  }

  main {
    width: min(620px, 94vw);
    margin: 88px auto 120px;
  }

  .tab-nav {
    display: none;
  }

  .mobile-nav {
    position: fixed;
    bottom: 12px;
    left: 50%;
    transform: translateX(-50%);
    display: flex;
    gap: 12px;
    background: rgba(255, 255, 255, 0.94);
    padding: 10px 14px;
    border-radius: 999px;
    box-shadow: 0 24px 45px rgba(15, 23, 42, 0.18);
    z-index: 12;
  }

  .mobile-nav .tab-button {
    padding: 10px 14px;
    font-size: 0.9rem;
  }

  .practice-panel {
    padding: 24px 18px;
  }

  .direction-controls {
    flex-direction: column;
    align-items: flex-start;
    gap: 12px;
  }

  .flashcard-wrapper {
    grid-template-columns: 1fr;
    padding: 16px;
    gap: 16px;
  }

  .nav-arrow {
    justify-self: center;
    width: 48px;
    height: 48px;
  }

  .flashcard {
    height: clamp(240px, 52vh, 360px);
  }

  .quiz-panel {
    padding: 20px;
  }

  .filter-bar {
    gap: 12px;
  }
}

@media (max-width: 640px) {
  main {
    margin: 72px auto 116px;
  }

  .card-grid {
    grid-template-columns: minmax(0, 1fr);
  }

  .filter-bar {
    flex-direction: column;
    align-items: stretch;
    padding: 16px;
  }

  .filter-group {
    width: 100%;
    flex-direction: column;
    align-items: flex-start;
    gap: 8px;
  }

  .direction-controls {
    width: 100%;
    gap: clamp(10px, 3vw, 16px);
    padding: 12px 0;
    border: none;
  }

  .segmented {
    width: 100%;
    display: grid;
    grid-template-columns: repeat(auto-fit, minmax(140px, 1fr));
    gap: 10px;
    padding: 0;
    background: transparent;
    border: none;
    box-shadow: none;
  }

  .segment-button {
    flex: unset;
    text-align: center;
    border-radius: calc(var(--radius) + 2px);
    padding: 12px 14px;
    border: 1px solid rgba(74, 107, 242, 0.2);
    background: #fff;
    box-shadow: 0 12px 20px rgba(15, 23, 42, 0.08);
  }

  .segment-button.active {
    box-shadow: 0 16px 28px rgba(74, 107, 242, 0.25);
  }
}

@media (max-width: 480px) {
  main {
    margin: 64px auto 110px;
  }

  .card {
    padding: 18px;
  }

  .segmented {
    grid-template-columns: 1fr;
  }
}

@keyframes fadeIn {
  from {
    opacity: 0;
    transform: translateY(14px);
  }
  to {
    opacity: 1;
    transform: translateY(0);
  }
}