*{margin:0;padding:0;box-sizing:border-box}body{font-family:-apple-system,BlinkMacSystemFont,Segoe UI,Roboto,Oxygen,Ubuntu,Cantarell,Fira Sans,Droid Sans,Helvetica Neue,sans-serif;-webkit-font-smoothing:antialiased;-moz-osx-font-smoothing:grayscale;background:#f5f7fa}.app{min-height:100vh;display:flex;flex-direction:column}.header{background:linear-gradient(135deg,#667eea,#764ba2);color:#fff;padding:1.5rem 2rem;box-shadow:0 2px 8px #0000001a;display:flex;justify-content:space-between;align-items:center}.header h1{font-size:2rem;font-weight:700}.back-btn{background:#fff3;border:none;color:#fff;padding:.75rem 1.5rem;border-radius:8px;font-size:1rem;cursor:pointer;transition:all .3s ease;font-weight:600}.back-btn:hover{background:#ffffff4d;transform:translateY(-2px)}.main{flex:1;padding:2rem;max-width:1400px;width:100%;margin:0 auto}.categories-grid h2,.subcategories h2,.lessons h2{font-size:1.75rem;margin-bottom:1.5rem;color:#2d3748}.grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(280px,1fr));gap:1.5rem;margin-top:1.5rem}.card{background:#fff;border-radius:12px;padding:2rem;box-shadow:0 4px 6px #00000012;cursor:pointer;transition:all .3s ease;border:2px solid transparent}.card:hover{transform:translateY(-4px);box-shadow:0 12px 24px #00000026;border-color:#667eea}.card h3{font-size:1.25rem;margin-bottom:.75rem;color:#2d3748}.card p{color:#718096;font-size:.95rem}.lessons-list{display:flex;flex-direction:column;gap:.75rem;margin-top:1.5rem}.lesson-item{background:#fff;padding:1.25rem 1.5rem;border-radius:10px;box-shadow:0 2px 4px #0000000d;cursor:pointer;transition:all .3s ease;display:flex;align-items:center;gap:1rem;border-left:4px solid transparent}.lesson-item:hover{transform:translate(8px);box-shadow:0 4px 12px #0000001a;border-left-color:#667eea}.lesson-number{background:linear-gradient(135deg,#667eea,#764ba2);color:#fff;width:32px;height:32px;border-radius:50%;display:flex;align-items:center;justify-content:center;font-weight:700;font-size:.9rem;flex-shrink:0}.lesson-title{color:#2d3748;font-size:1rem;font-weight:500}.lesson-viewer{width:100%}.lesson-viewer h2{font-size:1.75rem;margin-bottom:1.5rem;color:#2d3748}.iframe-container{width:100%;height:calc(100vh - 200px);background:#fff;border-radius:12px;overflow:hidden;box-shadow:0 8px 16px #0000001a}.iframe-container iframe{width:100%;height:100%;border:none}@media (max-width: 768px){.header{padding:1rem}.header h1{font-size:1.5rem}.main{padding:1rem}.grid{grid-template-columns:1fr}.iframe-container{height:calc(100vh - 180px)}}