/* ==========================================
   CSS Reset & Variables (디자인 시스템)
   ========================================== */
:root {
  /* Font Family */
  --font-main: 'Fredoka', 'Gowun Dodum', sans-serif;

  /* Theme: Pastel (Default) */
  --bg-gradient: linear-gradient(135deg, #fef9e7 0%, #e8f5e9 50%, #e0f7fa 100%);
  --bubble-color-1: rgba(255, 205, 210, 0.4);
  --bubble-color-2: rgba(224, 242, 241, 0.5);
  --bubble-color-3: rgba(255, 243, 224, 0.4);
  
  --panel-bg: rgba(255, 255, 255, 0.7);
  --panel-border: rgba(255, 255, 255, 0.5);
  --panel-shadow: 0 10px 30px rgba(121, 134, 203, 0.1);
  
  --card-bg-front: #ffffff;
  --card-bg-back: #ffffff;
  --card-shadow: 0 20px 40px rgba(0, 0, 0, 0.08);

  --text-main: #37474f;
  --text-muted: #78909c;
  --text-accent: #26a69a;
  
  --accent-color: #ffd54f; /* Yellow */
  --primary-color: #5c6bc0; /* Indigo */
  --primary-hover: #3f51b5;
  --success-color: #66bb6a; /* Green */
  --warning-color: #ffb74d; /* Orange */
  --danger-color: #ef5350; /* Red */
  
  --border-radius-lg: 24px;
  --border-radius-md: 16px;
  --border-radius-sm: 8px;
}

/* Theme: Dark */
body.dark-theme {
  --bg-gradient: linear-gradient(135deg, #0f172a 0%, #1e1b4b 50%, #111827 100%);
  --bubble-color-1: rgba(99, 102, 241, 0.15);
  --bubble-color-2: rgba(20, 184, 166, 0.1);
  --bubble-color-3: rgba(168, 85, 247, 0.15);
  
  --panel-bg: rgba(30, 41, 59, 0.7);
  --panel-border: rgba(255, 255, 255, 0.08);
  --panel-shadow: 0 10px 30px rgba(0, 0, 0, 0.4);
  
  --card-bg-front: #1e293b;
  --card-bg-back: #1e293b;
  --card-shadow: 0 20px 40px rgba(0, 0, 0, 0.5);

  --text-main: #f8fafc;
  --text-muted: #94a3b8;
  --text-accent: #38bdf8;
  
  --accent-color: #fbbf24;
  --primary-color: #6366f1;
  --primary-hover: #4f46e5;
  --success-color: #10b981;
  --warning-color: #f59e0b;
  --danger-color: #ef4444;
}

/* Theme: Light */
body.light-theme {
  --bg-gradient: linear-gradient(135deg, #f8fafc 0%, #f1f5f9 100%);
  --bubble-color-1: rgba(148, 163, 184, 0.05);
  --bubble-color-2: rgba(148, 163, 184, 0.05);
  --bubble-color-3: rgba(148, 163, 184, 0.05);
  
  --panel-bg: #ffffff;
  --panel-border: #e2e8f0;
  --panel-shadow: 0 4px 20px rgba(0, 0, 0, 0.05);
  
  --card-bg-front: #ffffff;
  --card-bg-back: #ffffff;
  --card-shadow: 0 15px 30px rgba(0, 0, 0, 0.06);

  --text-main: #1e293b;
  --text-muted: #64748b;
  --text-accent: #0f766e;
  
  --accent-color: #eab308;
  --primary-color: #2563eb;
  --primary-hover: #1d4ed8;
  --success-color: #16a34a;
  --warning-color: #ca8a04;
  --danger-color: #dc2626;
}

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

body {
  font-family: var(--font-main);
  background: var(--bg-gradient);
  color: var(--text-main);
  min-height: 100vh;
  overflow-x: hidden;
  position: relative;
  display: flex;
  justify-content: center;
  align-items: flex-start;
  padding: 20px;
  transition: background 0.5s ease, color 0.3s ease;
}

/* ==========================================
   Background Bubbles
   ========================================== */
.bg-bubble {
  position: fixed;
  border-radius: 50%;
  filter: blur(80px);
  z-index: -1;
  animation: floating 20s infinite alternate ease-in-out;
}

.bg-bubble-1 {
  width: 400px;
  height: 400px;
  background: var(--bubble-color-1);
  top: -100px;
  left: -100px;
}

.bg-bubble-2 {
  width: 500px;
  height: 500px;
  background: var(--bubble-color-2);
  bottom: -150px;
  right: -100px;
  animation-duration: 25s;
  animation-delay: -5s;
}

.bg-bubble-3 {
  width: 300px;
  height: 300px;
  background: var(--bubble-color-3);
  top: 40%;
  right: 15%;
  animation-duration: 18s;
  animation-delay: -2s;
}

@keyframes floating {
  0% { transform: translate(0, 0) scale(1); }
  50% { transform: translate(50px, 30px) scale(1.1); }
  100% { transform: translate(-30px, -50px) scale(0.9); }
}

/* ==========================================
   App Layout
   ========================================== */
.app-container {
  width: 100%;
  max-width: 1200px;
  display: flex;
  flex-direction: column;
  gap: 20px;
}

/* Header */
.app-header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  background: var(--panel-bg);
  backdrop-filter: blur(16px);
  -webkit-backdrop-filter: blur(16px);
  border: 1px solid var(--panel-border);
  padding: 16px 28px;
  border-radius: var(--border-radius-md);
  box-shadow: var(--panel-shadow);
  z-index: 10;
}

.logo-area {
  display: flex;
  align-items: center;
  gap: 12px;
}

.logo-icon {
  color: var(--warning-color);
  width: 32px;
  height: 32px;
  animation: rotateSparkle 4s infinite linear;
}

@keyframes rotateSparkle {
  0% { transform: rotate(0deg) scale(1); }
  50% { transform: rotate(180deg) scale(1.2); }
  100% { transform: rotate(360deg) scale(1); }
}

.logo-area h1 {
  font-size: 26px;
  font-weight: 700;
  letter-spacing: -0.5px;
}

.logo-area h1 span {
  color: var(--primary-color);
}

.header-controls {
  display: flex;
  align-items: center;
  gap: 12px;
}

/* Buttons */
.primary-btn, .secondary-btn, .icon-btn, .action-btn, .danger-btn {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  gap: 8px;
  border: none;
  font-family: var(--font-main);
  font-size: 15px;
  font-weight: 600;
  cursor: pointer;
  border-radius: var(--border-radius-sm);
  transition: all 0.25s cubic-bezier(0.4, 0, 0.2, 1);
  outline: none;
}

.primary-btn {
  background: var(--primary-color);
  color: #ffffff;
  padding: 12px 20px;
  box-shadow: 0 4px 12px rgba(99, 102, 241, 0.2);
}

.primary-btn:hover {
  background: var(--primary-hover);
  transform: translateY(-2px);
  box-shadow: 0 6px 16px rgba(99, 102, 241, 0.35);
}

.secondary-btn {
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  color: var(--text-main);
  padding: 12px 18px;
}

.secondary-btn:hover {
  background: var(--text-main);
  color: var(--card-bg-front);
  transform: translateY(-2px);
}

.icon-btn {
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  color: var(--text-main);
  width: 44px;
  height: 44px;
  border-radius: 50%;
}

.icon-btn:hover {
  background: var(--primary-color);
  color: white;
  transform: scale(1.08);
}

.danger-btn {
  background: var(--danger-color);
  color: white;
  padding: 8px 12px;
}

.danger-btn:hover {
  opacity: 0.9;
  transform: scale(0.97);
}

/* ==========================================
   Views (Study & Manager)
   ========================================== */
.view-section {
  display: none;
  animation: fadeIn 0.4s ease forwards;
}

.view-section.active {
  display: block;
}

@keyframes fadeIn {
  from { opacity: 0; transform: translateY(15px); }
  to { opacity: 1; transform: translateY(0); }
}

/* Study View Elements */
.study-stats {
  display: flex;
  justify-content: space-between;
  align-items: center;
  flex-wrap: wrap;
  gap: 16px;
  margin-bottom: 24px;
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  padding: 16px 24px;
  border-radius: var(--border-radius-md);
  box-shadow: var(--panel-shadow);
}

.progress-container {
  display: flex;
  flex-direction: column;
  gap: 6px;
  flex-grow: 1;
  max-width: 500px;
}

.progress-text {
  font-size: 14px;
  color: var(--text-muted);
}

.progress-bar-bg {
  width: 100%;
  height: 12px;
  background: rgba(0, 0, 0, 0.05);
  border-radius: 6px;
  overflow: hidden;
  position: relative;
}

.progress-bar-fill {
  height: 100%;
  background: var(--success-color);
  border-radius: 6px;
  transition: width 0.4s cubic-bezier(0.4, 0, 0.2, 1);
}

.deck-selector-wrapper {
  display: flex;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
}

.select-group {
  display: flex;
  align-items: center;
  gap: 6px;
}

.select-group label {
  font-weight: 700;
  font-size: 14px;
  display: flex;
  align-items: center;
  gap: 4px;
}

.custom-select {
  padding: 8px 14px;
  font-family: var(--font-main);
  font-size: 14px;
  font-weight: 700;
  border: 1.5px solid var(--panel-border);
  border-radius: var(--border-radius-sm);
  background: var(--card-bg-front);
  color: var(--text-main);
  outline: none;
  cursor: pointer;
  transition: all 0.3s;
}

.custom-select:focus {
  border-color: var(--primary-color);
  box-shadow: 0 0 0 3px rgba(99, 102, 241, 0.1);
}

/* ==========================================
   3D Card Mechanism
   ========================================== */
.card-display-container {
  display: flex;
  align-items: center;
  justify-content: center;
  gap: 32px;
  margin: 32px 0;
  position: relative;
}

.nav-btn {
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  color: var(--text-main);
  width: 60px;
  height: 60px;
  border-radius: 50%;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  box-shadow: var(--panel-shadow);
  transition: all 0.3s ease;
  z-index: 5;
}

.nav-btn:hover {
  background: var(--primary-color);
  color: #ffffff;
  transform: translateY(-4px) scale(1.05);
}

/* 3D Wrapper */
.card-3d-wrapper {
  width: 100%;
  max-width: 480px;
  height: 380px;
  perspective: 1200px;
  cursor: pointer;
  position: relative;
}

/* Card Faces */
.card-face {
  position: absolute;
  width: 100%;
  height: 100%;
  border-radius: var(--border-radius-lg);
  padding: 24px;
  display: flex;
  flex-direction: column;
  justify-content: space-between;
  backface-visibility: hidden;
  -webkit-backface-visibility: hidden;
  transition: transform 0.6s cubic-bezier(0.175, 0.885, 0.32, 1.275), box-shadow 0.6s;
  box-shadow: var(--card-shadow);
  border: 2px solid var(--panel-border);
}

.card-front {
  background: var(--card-bg-front);
  transform: rotateY(0deg);
}

.card-back {
  background: var(--card-bg-back);
  transform: rotateY(180deg);
}

/* Flip state logic */
.card-3d-wrapper.is-flipped .card-front {
  transform: rotateY(-180deg);
}

.card-3d-wrapper.is-flipped .card-back {
  transform: rotateY(0deg);
}

/* Card Inner Components */
.card-header-actions {
  display: flex;
  justify-content: space-between;
  align-items: center;
  width: 100%;
}

.card-index-indicator {
  font-size: 13px;
  font-weight: 700;
  color: var(--text-muted);
  background: rgba(0, 0, 0, 0.05);
  padding: 4px 10px;
  border-radius: 12px;
}

.tts-btn {
  background: rgba(99, 102, 241, 0.1);
  border: none;
  color: var(--primary-color);
  width: 38px;
  height: 38px;
  border-radius: 50%;
  cursor: pointer;
  display: flex;
  align-items: center;
  justify-content: center;
  transition: all 0.2s;
}

.tts-btn:hover {
  background: var(--primary-color);
  color: white;
  transform: scale(1.1);
}

.back-audio-controls {
  display: flex;
  gap: 8px;
}

/* Card Front Image */
.card-image-box {
  flex-grow: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  margin: 16px 0;
  border-radius: var(--border-radius-md);
  position: relative;
  background: rgba(0, 0, 0, 0.02);
  border: 1px dashed var(--panel-border);
}

.card-image-box img {
  width: 100%;
  height: 100%;
  object-fit: cover;
  border-radius: var(--border-radius-md);
  animation: zoomIn 0.5s ease;
}

@keyframes zoomIn {
  from { transform: scale(0.9); opacity: 0; }
  to { transform: scale(1); opacity: 1; }
}

.image-fallback {
  display: flex;
  flex-direction: column;
  align-items: center;
  gap: 12px;
  color: var(--text-muted);
}

.image-fallback i {
  width: 48px;
  height: 48px;
  opacity: 0.6;
}

.image-fallback span {
  font-size: 14px;
}

.card-front-hint {
  text-align: center;
}

.pulse-text {
  font-size: 13px;
  color: var(--text-muted);
  font-weight: 500;
  animation: pulse 2s infinite;
}

@keyframes pulse {
  0% { opacity: 0.5; }
  50% { opacity: 1; }
  100% { opacity: 0.5; }
}

/* Card Back Details */
.card-detail-content {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  flex-grow: 1;
  text-align: center;
  padding: 10px 0;
}

.word-text {
  font-size: 40px;
  font-weight: 700;
  letter-spacing: -1px;
  color: var(--primary-color);
}

.phonetic-text {
  font-size: 16px;
  color: var(--text-muted);
  margin-top: 4px;
}

.divider {
  width: 60px;
  height: 4px;
  background: var(--warning-color);
  border-radius: 2px;
  margin: 16px 0;
}

.meaning-kr {
  font-size: 24px;
  font-weight: 600;
  color: var(--text-main);
  margin-bottom: 8px;
}

.meaning-en {
  font-size: 13px;
  color: var(--text-muted);
  line-height: 1.5;
  max-width: 90%;
  margin-bottom: 16px;
}

.sentence-box {
  width: 100%;
  background: rgba(0, 0, 0, 0.02);
  border-left: 4px solid var(--accent-color);
  padding: 12px;
  border-radius: 0 var(--border-radius-sm) var(--border-radius-sm) 0;
  text-align: left;
}

.sentence-box .badge {
  display: inline-block;
  font-size: 10px;
  font-weight: 700;
  background: var(--accent-color);
  color: var(--text-main);
  padding: 2px 6px;
  border-radius: 4px;
  margin-bottom: 6px;
}

.example-text {
  font-size: 14px;
  line-height: 1.4;
  color: var(--text-main);
  font-style: italic;
}

/* Study Control Buttons */
.study-actions {
  display: flex;
  justify-content: center;
  align-items: center;
  gap: 16px;
  flex-wrap: wrap;
}

.action-btn {
  padding: 16px 28px;
  font-size: 16px;
  border-radius: var(--border-radius-md);
  box-shadow: 0 4px 10px rgba(0, 0, 0, 0.05);
}

.action-btn.success {
  background: var(--success-color);
  color: white;
}

.action-btn.warning {
  background: var(--warning-color);
  color: white;
}

.action-btn.danger {
  background: var(--panel-bg);
  border: 1px solid var(--panel-border);
  color: var(--danger-color);
  padding: 16px 20px;
}

.action-btn:hover {
  transform: translateY(-3px);
  box-shadow: 0 8px 20px rgba(0, 0, 0, 0.12);
}

.action-btn.danger:hover {
  background: var(--danger-color);
  color: white;
}

/* ==========================================
   Spinners & Toast Notifications
   ========================================== */
.spinner {
  border: 4px solid rgba(0, 0, 0, 0.05);
  border-top-color: var(--primary-color);
  border-radius: 50%;
  width: 36px;
  height: 36px;
  animation: spin 1s linear infinite;
}

@keyframes spin {
  to { transform: rotate(360deg); }
}

.toast-container {
  position: fixed;
  bottom: 24px;
  right: 24px;
  display: flex;
  flex-direction: column;
  gap: 8px;
  z-index: 1000;
}

.toast {
  background: var(--panel-bg);
  backdrop-filter: blur(16px);
  border: 1px solid var(--panel-border);
  color: var(--text-main);
  padding: 12px 24px;
  border-radius: var(--border-radius-sm);
  box-shadow: 0 8px 16px rgba(0, 0, 0, 0.15);
  display: flex;
  align-items: center;
  gap: 8px;
  font-size: 14px;
  font-weight: 600;
  animation: slideIn 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275) forwards;
}

.toast.success {
  border-left: 4px solid var(--success-color);
}

.toast.error {
  border-left: 4px solid var(--danger-color);
}

.toast.info {
  border-left: 4px solid var(--primary-color);
}

@keyframes slideIn {
  from { transform: translateX(100%); opacity: 0; }
  to { transform: translateX(0); opacity: 1; }
}

/* Custom Scrollbar */
::-webkit-scrollbar {
  width: 6px;
}

::-webkit-scrollbar-track {
  background: transparent;
}

::-webkit-scrollbar-thumb {
  background: var(--panel-border);
  border-radius: 3px;
}

::-webkit-scrollbar-thumb:hover {
  background: var(--text-muted);
}

/* ==========================================
   Responsive Design
   ========================================== */
@media (max-width: 768px) {
  body {
    padding: 10px;
  }
  
  .app-header {
    padding: 12px 16px;
  }
  
  .logo-area h1 {
    font-size: 20px;
  }
  
  .card-display-container {
    gap: 12px;
  }
  
  .nav-btn {
    width: 48px;
    height: 48px;
  }
  
  .card-3d-wrapper {
    height: 340px;
  }
  
  .word-text {
    font-size: 32px;
  }
  
  .meaning-kr {
    font-size: 20px;
  }
  
  .study-stats {
    flex-direction: column;
    align-items: stretch;
  }
}