*,*::before,*::after{box-sizing:border-box}html{scroll-behavior:smooth;scroll-padding-top:calc(var(--header-h) + 16px)}body{margin:0;font-family:var(--font-main);font-size:16px;line-height:1.8;color:var(--color-text);background:var(--color-bg);overflow-x:hidden}a{color:var(--color-primary);text-decoration:none}p{color:var(--color-muted);margin:0 0 1em}h1,h2,h3{color:var(--color-secondary);line-height:1.35;margin:0 0 .6em}code{direction:ltr;display:inline-block;background:var(--color-primary-tint);color:var(--color-secondary);border-radius:6px;padding:0 6px}.container{width:100%;max-width:var(--container);margin:0 auto;padding:0 24px}.navbar{position:sticky;top:0;z-index:1000;height:var(--header-h);display:flex;align-items:center;background:rgba(255,255,255,.96);backdrop-filter:blur(12px);border-bottom:1px solid var(--color-border)}.nav-container{width:100%;max-width:var(--container);margin:0 auto;padding:0 24px;display:flex;align-items:center;justify-content:space-between;gap:16px}.nav-brand{display:flex;align-items:center;gap:10px;color:var(--color-secondary);font-weight:800}.brand-icon{width:38px;height:38px;display:inline-flex;align-items:center;justify-content:center;border-radius:10px;background:linear-gradient(135deg,var(--color-primary),var(--color-secondary));color:#fff}.nav-menu{display:flex;align-items:center;gap:4px}.nav-link{color:var(--color-muted);font-weight:700;font-size:.88rem;padding:7px 11px;border-radius:var(--border-radius-sm);white-space:nowrap}.nav-link:hover,.nav-link.active{background:var(--color-primary-tint);color:var(--color-secondary)}.nav-btn{background:linear-gradient(135deg,var(--color-primary),var(--color-secondary));color:#fff!important;box-shadow:var(--shadow-purple);margin-right:6px}.nav-toggle{display:none;width:42px;height:42px;border:0;border-radius:10px;background:var(--color-primary-tint);color:var(--color-secondary);font-size:1.2rem}.page-hero{background:linear-gradient(135deg,var(--color-footer) 0%,var(--color-secondary) 55%,var(--color-primary) 100%);color:#fff;padding:56px 0 52px;position:relative;overflow:hidden}.page-hero::before{content:"";position:absolute;inset:0;opacity:.3;background:radial-gradient(circle at 10% 30%,rgba(245,158,11,.2) 0,transparent 40%),radial-gradient(circle at 90% 70%,rgba(167,139,250,.25) 0,transparent 40%)}.page-hero .container{position:relative}.breadcrumb{display:flex;gap:8px;flex-wrap:wrap;color:rgba(255,255,255,.72);font-size:.9rem;margin-bottom:14px}.breadcrumb a{color:rgba(255,255,255,.9)}.series-badge{display:inline-flex;background:rgba(255,255,255,.12);color:#FCD34D;border:1px solid rgba(255,255,255,.2);border-radius:999px;padding:4px 12px;font-weight:800;font-size:.82rem;margin-bottom:14px}.page-hero-title{font-size:clamp(1.7rem,4vw,2.6rem);font-weight:900;color:#fff}.page-hero-desc{max-width:780px;color:rgba(255,255,255,.9);font-size:1.05rem}.section{padding:64px 0}.soft-bg{background:var(--color-surface)}.section-header{text-align:center;margin-bottom:40px}.section-title{font-size:clamp(1.45rem,3vw,2rem);font-weight:900}.section-title::after{content:"";display:block;width:56px;height:4px;margin:12px auto 0;border-radius:2px;background:linear-gradient(90deg,var(--color-primary),var(--color-accent))}.section-sub{max-width:740px;margin:0 auto}.grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(280px,1fr));gap:20px}.grid.three{grid-template-columns:repeat(3,1fr)}.card,.mini-card{background:#fff;border:1px solid var(--color-border);border-radius:var(--border-radius-lg);padding:26px 22px;box-shadow:var(--shadow-card);transition:.25s ease;position:relative;overflow:hidden}.card::before{content:"";position:absolute;top:0;right:0;left:0;height:4px;background:linear-gradient(90deg,var(--color-primary),var(--color-accent));transform:scaleX(0);transform-origin:right;transition:.25s ease}.card:hover{transform:translateY(-6px);box-shadow:var(--shadow-hover);border-color:var(--color-primary-light)}.card:hover::before{transform:scaleX(1)}.card-icon{width:54px;height:54px;border-radius:var(--border-radius);display:flex;align-items:center;justify-content:center;background:linear-gradient(135deg,var(--color-primary-tint),#fff);border:1px solid #F3F4F6;color:var(--color-secondary);font-size:1.4rem;margin-bottom:14px}.card h3{font-size:1.05rem;font-weight:800}.card p{font-size:.94rem}.card-btn,.btn{display:inline-flex;align-items:center;gap:8px;padding:10px 16px;border-radius:var(--border-radius-sm);font-weight:800}.card-btn{background:var(--color-primary-tint);color:var(--color-secondary)}.btn-primary{background:var(--color-accent);color:#111827;box-shadow:0 8px 20px -6px rgba(245,158,11,.5)}.article-shell{max-width:1200px;margin:0 auto;padding:42px 24px 70px;display:grid;grid-template-columns:260px minmax(0,900px);gap:32px;align-items:start}.toc{position:sticky;top:92px;background:#fff;border:1px solid var(--color-border);border-radius:var(--border-radius);box-shadow:var(--shadow-card);padding:18px}.toc a{display:block;color:var(--color-muted);padding:7px 0;border-top:1px solid #F3F4F6}.content-area{max-width:900px;margin:0 auto;padding:40px 24px 64px}.content-area h2{font-size:1.3rem;font-weight:900;margin:30px 0 10px;padding-right:14px;border-right:4px solid var(--color-primary)}.content-area ul,.content-area ol{margin:0 0 18px;padding-right:22px;color:var(--color-muted)}.highlight-box,.note-box{background:var(--color-primary-tint);border:1px solid var(--color-primary-light);border-right:4px solid var(--color-primary);border-radius:var(--border-radius);padding:18px 20px;margin:20px 0;color:var(--color-secondary)}.notice{background:var(--color-primary-tint);border:1px solid var(--color-primary-light);border-radius:var(--border-radius);padding:18px 20px;text-align:center;font-weight:800;color:var(--color-secondary)}.mistake-box{background:#FEF2F2;border:1px solid #FECACA;border-right:4px solid #DC2626;border-radius:var(--border-radius);padding:18px 20px;margin:20px 0;color:#7F1D1D}.tip-box{background:#ECFDF5;border:1px solid #A7F3D0;border-right:4px solid #059669;border-radius:var(--border-radius);padding:18px 20px;margin:20px 0;color:#064E3B}.example-box{background:var(--color-surface);border:1px solid var(--color-border);border-radius:var(--border-radius);padding:18px 20px;margin:20px 0}.keyword-list{display:flex;flex-wrap:wrap;gap:8px;list-style:none!important;padding-right:0!important}.related-links{display:grid;grid-template-columns:repeat(auto-fill,minmax(230px,1fr));gap:10px}.related-links a{background:var(--color-surface);border:1px solid var(--color-border);border-radius:var(--border-radius-sm);padding:12px 14px;color:var(--color-muted);font-weight:700}.faq-item{background:#fff;border:1px solid var(--color-border);border-radius:var(--border-radius);margin-bottom:12px;box-shadow:var(--shadow-card);overflow:hidden}.faq-item summary{cursor:pointer;padding:15px 18px;font-weight:900;color:var(--color-secondary)}.faq-item p{padding:0 18px 16px;margin:0}.site-footer{background:var(--color-footer);color:rgba(255,255,255,.85);padding:48px 0 28px;margin-top:40px}.footer-inner{text-align:center}.footer-brand{display:inline-flex;align-items:center;gap:8px;color:#fff;font-weight:800;margin-bottom:12px}.footer-desc{color:rgba(255,255,255,.7)}.footer-links{display:flex;gap:8px 16px;justify-content:center;flex-wrap:wrap;margin-bottom:20px}.footer-links a{color:rgba(255,255,255,.78)}.footer-copy{color:rgba(255,255,255,.45);border-top:1px solid rgba(255,255,255,.1);padding-top:18px}@media(max-width:900px){.article-shell{grid-template-columns:1fr}.toc{position:static}.nav-link{font-size:.82rem;padding:6px 8px}.grid.three{grid-template-columns:1fr}}@media(max-width:720px){.container,.nav-container{padding:0 18px}.nav-menu{display:none;position:absolute;top:var(--header-h);right:0;left:0;background:#fff;border-bottom:1px solid var(--color-border);padding:16px 20px;flex-direction:column;align-items:stretch;box-shadow:var(--shadow-hover)}.nav-menu.open{display:flex}.nav-toggle{display:flex;align-items:center;justify-content:center}.page-hero{padding:44px 0 40px}.section{padding:46px 0}.grid{grid-template-columns:1fr}.article-shell,.content-area{padding:30px 18px 54px}}@media(max-width:480px){.nav-brand span:last-child{display:none}.page-hero-title{font-size:1.6rem}}

/* ===== FE exams and sharing additions ===== */
.share-panel{display:flex;align-items:center;gap:10px;flex-wrap:wrap;background:var(--color-primary-tint);border:1px solid var(--color-primary-light);border-radius:var(--border-radius);padding:14px 16px;margin:0 0 22px;color:var(--color-secondary)}
.share-panel button{font-family:var(--font-main);border:1px solid var(--color-primary-light);background:#fff;color:var(--color-secondary);border-radius:var(--border-radius-sm);padding:8px 12px;font-weight:800;cursor:pointer}
.share-panel button:hover{background:var(--color-primary);color:#fff}
.share-panel span{color:var(--color-muted);font-size:.9rem}
.footer-social{display:flex;gap:8px 12px;justify-content:center;flex-wrap:wrap;margin:0 0 18px}
.footer-social a{color:#fff;background:rgba(255,255,255,.12);border:1px solid rgba(255,255,255,.18);border-radius:999px;padding:6px 12px;font-weight:800;font-size:.9rem}
.exam-page{padding-top:34px}
.exam-instructions{background:#fff;border:1px solid var(--color-border);border-radius:var(--border-radius);box-shadow:var(--shadow-card);padding:18px 20px;margin:20px 0}
.question-card{background:#fff;border:1px solid var(--color-border);border-radius:var(--border-radius-lg);box-shadow:var(--shadow-card);padding:22px;margin:18px 0}
.question-card h2{margin-top:0!important}
.choice-list{display:grid;grid-template-columns:repeat(2,minmax(0,1fr));gap:10px;list-style:none!important;padding:0!important;margin:12px 0!important;counter-reset:choice}
.choice-list li{background:var(--color-surface);border:1px solid var(--color-border);border-radius:var(--border-radius-sm);padding:10px 12px;color:var(--color-muted)}
.choice-list li::before{counter-increment:choice;content:counter(choice, upper-alpha) ". ";font-weight:900;color:var(--color-secondary)}
.answer-box{border:1px solid #A7F3D0;background:#ECFDF5;border-radius:var(--border-radius);padding:12px 14px;margin-top:12px;color:#064E3B}
.answer-box summary{cursor:pointer;font-weight:900;color:#065F46}
@media(max-width:720px){.choice-list{grid-template-columns:1fr}.share-panel{align-items:stretch}.share-panel button{flex:1}}


/* ===== FE book popup ===== */
.fe-book-popup{position:fixed;inset:0;z-index:3000;display:none;align-items:center;justify-content:center;padding:22px;background:rgba(17,24,39,.48);backdrop-filter:blur(6px)}
.fe-book-popup.show{display:flex}
.fe-book-popup-card{width:min(100%,460px);background:#fff;border:1px solid var(--color-border);border-radius:var(--border-radius-lg);box-shadow:0 24px 70px -24px rgba(59,7,100,.5);padding:28px 24px;position:relative;text-align:center;overflow:hidden}
.fe-book-popup-card::before{content:"";position:absolute;top:0;right:0;left:0;height:5px;background:linear-gradient(90deg,var(--color-primary),var(--color-accent))}
.fe-book-popup-close{position:absolute;top:12px;left:12px;width:36px;height:36px;border:0;border-radius:50%;background:var(--color-primary-tint);color:var(--color-secondary);font-size:1.35rem;line-height:1;cursor:pointer}
.fe-book-popup-kicker{display:inline-flex;align-items:center;justify-content:center;margin-bottom:12px;padding:4px 12px;border-radius:999px;background:var(--color-primary-tint);color:var(--color-secondary);font-weight:900;font-size:.82rem}
.fe-book-popup h2{font-size:1.55rem;margin:0 0 10px;color:var(--color-secondary)}
.fe-book-popup p{margin:0 auto 18px;max-width:360px;color:var(--color-muted)}
.fe-book-popup-cta{display:inline-flex;align-items:center;justify-content:center;gap:8px;border-radius:var(--border-radius-sm);padding:12px 20px;background:var(--color-accent);color:#111827;font-weight:900;box-shadow:0 8px 20px -6px rgba(245,158,11,.5)}
.fe-book-popup-cta:hover{background:#D97706;color:#fff}
@media(max-width:480px){.fe-book-popup{padding:16px}.fe-book-popup-card{padding:26px 18px}.fe-book-popup h2{font-size:1.3rem}}