:root{--bg: #f7f9fa;--surface: #ffffff;--ink: #1c2b30;--muted: #5d6e74;--accent: #0f8a8a;--accent-deep: #0a6b6b;--accent-soft: #e1f5ee;--accent-tint: #eef7f7;--gold: #c9842a;--gold-soft: #f8efdd;--border: #e1e7e9;--ok: #1c7a4d;--err: #b23a3a;--radius: 10px;font-family:Inter,system-ui,-apple-system,sans-serif;font-variant-numeric:tabular-nums}*{box-sizing:border-box}body{margin:0;background:var(--bg);color:var(--ink)}.app{max-width:900px;margin:0 auto;padding:24px}.app__header{display:flex;align-items:center;justify-content:space-between;gap:16px;margin-bottom:24px}.tagline{color:var(--muted);margin:4px 0 0}.lang-switch button{border:1px solid var(--border);background:var(--surface);padding:6px 10px;border-radius:8px;cursor:pointer;margin-left:6px}.lang-switch button[aria-pressed=true]{border-color:var(--accent);color:var(--accent);font-weight:600}.app__main{display:grid;gap:24px}.calculator,.problem{background:var(--surface);border:1px solid var(--border);border-radius:var(--radius);padding:20px}.subtitle{color:var(--muted);margin-top:0}.calculator__form{display:grid;gap:12px;max-width:360px}label{display:grid;gap:4px;font-size:14px;color:var(--muted)}input,select{padding:8px 10px;border:1px solid var(--border);border-radius:8px;font:inherit;color:var(--ink);background:var(--surface)}button[type=submit],.problem__feedback button{justify-self:start;background:var(--accent);color:#fff;border:none;padding:9px 16px;border-radius:8px;cursor:pointer}.calculator__result,.problem__feedback{margin-top:16px}.calculator__error{color:var(--err)}.solution-steps{color:var(--ink);line-height:1.7;padding-left:20px}.is-correct{color:var(--ok);font-weight:600}.is-incorrect{color:var(--err);font-weight:600}.layout{min-height:100vh;display:flex;flex-direction:column}.layout__header{position:sticky;top:0;z-index:10;display:flex;align-items:center;gap:20px;padding:12px 24px;background:#f7f9fae6;-webkit-backdrop-filter:blur(6px);backdrop-filter:blur(6px);border-bottom:1px solid var(--border)}.brand{font-size:19px;font-weight:600;color:var(--ink);text-decoration:none;letter-spacing:-.01em}.layout__nav{display:flex;gap:16px}.layout__nav a{color:var(--muted);text-decoration:none;font-size:14px;padding:4px 2px}.layout__nav a.active,.layout__nav a:hover{color:var(--accent)}.layout__right{margin-left:auto}.layout__main{flex:1;width:100%;max-width:960px;margin:0 auto;padding:28px 24px 48px}.layout__footer{border-top:1px solid var(--border);color:var(--muted);font-size:13px;padding:18px 24px;text-align:center}.page-head{margin-bottom:18px}.page-head h1{font-size:24px;font-weight:600;margin:0}.catalog__search{display:flex;align-items:center;gap:8px;max-width:360px;border:1px solid var(--border);border-radius:10px;background:var(--surface);padding:0 12px;color:var(--muted);margin-bottom:8px}.catalog__search input{border:none;background:transparent;padding:9px 0;flex:1}.catalog__group{margin-top:26px}.catalog__group-title{font-size:13px;font-weight:600;letter-spacing:.04em;text-transform:uppercase;color:var(--muted);margin:0 0 12px}.catalog__grid{display:grid;grid-template-columns:repeat(auto-fill,minmax(190px,1fr));gap:12px}.topic-card{position:relative;display:flex;flex-direction:column;gap:4px;background:var(--surface);border:1px solid var(--border);border-radius:12px;padding:16px;text-decoration:none;color:var(--ink);transition:border-color .12s,transform .12s}.topic-card:hover{border-color:var(--accent);transform:translateY(-1px)}.topic-card__icon{color:var(--accent);margin-bottom:6px}.topic-card__title{font-size:15px;font-weight:500;padding-right:22px}.topic-card__tagline{font-size:12.5px;color:var(--muted)}.catalog__group--fav .topic-card{border-color:var(--accent);background:linear-gradient(180deg,var(--accent-tint) 0%,var(--surface) 70%)}.fav-btn{display:inline-flex;align-items:center;justify-content:center;background:transparent;border:none;cursor:pointer;padding:2px;color:var(--muted);border-radius:6px;line-height:0}.fav-btn:hover,.fav-btn.is-fav{color:var(--gold)}.topic-card__fav{position:absolute;top:10px;right:10px}.module__fav{margin-left:auto}.breadcrumb{display:flex;align-items:center;gap:6px;font-size:13px;color:var(--muted);margin-bottom:14px}.breadcrumb a{color:var(--muted);text-decoration:none}.breadcrumb a:hover{color:var(--accent)}.breadcrumb__current{color:var(--ink)}.module>.calculator{margin-top:14px}.module__practice{margin-top:18px}.module__practice-title{display:flex;align-items:center;gap:6px;font-size:16px;font-weight:600;margin:0 0 8px}.explain{border:1px solid var(--border);border-radius:12px;overflow:hidden;background:var(--surface)}.explain__head{width:100%;display:flex;align-items:center;justify-content:space-between;background:#eef4f4;border:none;padding:11px 14px;cursor:pointer;font:inherit}.explain__title{display:flex;align-items:center;gap:7px;font-size:14px;font-weight:500;color:var(--ink)}.explain__toggle{display:flex;align-items:center;gap:5px;font-size:13px;color:var(--accent)}.explain__toggle .is-open{transform:rotate(180deg)}.explain__body{padding:12px 14px;border-top:1px solid var(--border)}.explain__body p{margin:0;font-size:14px;line-height:1.65;color:var(--ink)}.explain__hint{margin-top:8px!important;font-size:12px;color:var(--muted)}.practice{background:var(--surface);border:1px solid var(--border);border-radius:12px;padding:16px 18px}.practice__prompt{font-size:15px;line-height:1.55;margin:0 0 12px}.practice__form{display:flex;align-items:flex-end;gap:10px;flex-wrap:wrap}.practice__form label{flex:1;min-width:160px}.practice__form button[type=submit]{background:var(--accent);color:#fff;border:none;padding:9px 16px;border-radius:8px;cursor:pointer}.practice__feedback{margin-top:14px}button.ghost{display:inline-flex;align-items:center;gap:6px;margin-top:12px;background:transparent;border:1px solid var(--border);color:var(--ink);padding:8px 13px;border-radius:8px;cursor:pointer}.landing{display:flex;flex-direction:column;gap:48px}.hero{text-align:center;padding:44px 20px 28px;border-radius:22px;background:radial-gradient(1100px 320px at 50% -40px,var(--accent-tint) 0%,transparent 70%),linear-gradient(180deg,#fbfdfd 0%,var(--bg) 100%);border:1px solid var(--border)}.hero__badge{display:inline-flex;align-items:center;gap:6px;font-size:12.5px;font-weight:600;color:var(--accent-deep);background:var(--accent-soft);border-radius:999px;padding:5px 13px;margin-bottom:16px}.hero__title{font-size:38px;line-height:1.14;font-weight:700;letter-spacing:-.02em;margin:0}.hero__accent{background:linear-gradient(95deg,var(--accent) 0%,#16a9a0 55%,#2bb3a0 100%);-webkit-background-clip:text;background-clip:text;color:transparent}.hero__tagline{font-size:18px;color:var(--ink);margin:14px auto 0;font-weight:600}.hero__subtitle{max-width:560px;margin:10px auto 0;color:var(--muted);font-size:16px;line-height:1.6}.hero__cta{display:flex;gap:10px;justify-content:center;margin-top:24px;flex-wrap:wrap}.btn{display:inline-flex;align-items:center;gap:7px;padding:11px 18px;border-radius:10px;font-size:15px;font-weight:500;text-decoration:none;cursor:pointer;transition:transform .15s,box-shadow .15s}.btn--primary{background:var(--accent);color:#fff;box-shadow:0 8px 20px #0f8a8a38}.btn--primary:hover{transform:translateY(-1px);box-shadow:0 10px 26px #0f8a8a4d}.btn--ghost{background:var(--surface);color:var(--ink);border:1px solid var(--border)}.btn--ghost:hover{border-color:var(--accent);color:var(--accent)}.features{display:grid;grid-template-columns:repeat(auto-fit,minmax(220px,1fr));gap:16px}.feature{background:var(--surface);border:1px solid var(--border);border-radius:14px;padding:20px;transition:transform .15s,box-shadow .15s,border-color .15s}.feature:hover{transform:translateY(-2px);box-shadow:0 10px 26px #14282d14}.feature__icon{display:inline-flex;align-items:center;justify-content:center;width:42px;height:42px;border-radius:12px;background:var(--accent-soft);color:var(--accent)}.feature h3{font-size:16px;font-weight:600;margin:12px 0 4px}.feature p{margin:0;color:var(--muted);font-size:14px;line-height:1.55}.landing__modules{text-align:center}.landing__modules h2{font-size:22px;font-weight:600;margin:0}.landing__chips{display:flex;flex-wrap:wrap;gap:8px;justify-content:center;margin-top:16px}.mod-chip{display:inline-flex;align-items:center;gap:6px;padding:7px 12px;border:1px solid var(--border);border-radius:999px;background:var(--surface);color:var(--ink);text-decoration:none;font-size:13px;transition:all .14s}.mod-chip svg{color:var(--accent)}.mod-chip:hover{border-color:var(--accent);color:var(--accent);background:var(--accent-tint)}.tiers{text-align:center}.tiers h2{font-size:22px;font-weight:600;margin:0}.tiers__trial{display:inline-block;color:var(--accent-deep);background:var(--accent-soft);font-size:13.5px;font-weight:500;padding:6px 14px;border-radius:999px;margin:10px 0 20px}.tiers__grid{display:grid;grid-template-columns:repeat(auto-fit,minmax(240px,1fr));gap:16px;text-align:left;align-items:start}.tier{position:relative;background:var(--surface);border:1px solid var(--border);border-radius:14px;padding:22px}.tier--pro{border:2px solid var(--accent);background:linear-gradient(180deg,var(--accent-tint) 0%,var(--surface) 60%);box-shadow:0 16px 40px #0f8a8a29}.tier__badge{position:absolute;top:-12px;right:18px;display:inline-flex;align-items:center;gap:5px;background:var(--gold);color:#fff;font-size:12px;font-weight:700;padding:4px 12px;border-radius:999px;box-shadow:0 4px 12px #c9842a59}.tier h3{margin:0 0 6px;font-size:18px;font-weight:700}.tier--pro h3{color:var(--accent-deep)}.tier p{margin:0;color:var(--muted);font-size:14px;line-height:1.6}.layout__right{display:flex;align-items:center;gap:12px}.auth__signin{font-size:13px;color:var(--accent);text-decoration:none;border:1px solid var(--border);border-radius:999px;padding:5px 12px}.auth__signin:hover{border-color:var(--accent)}.auth__user{display:flex;align-items:center;gap:8px}.auth__chip{font-size:12px;padding:3px 10px;border-radius:999px;background:#e1f5ee;color:var(--accent)}.auth__signout{display:inline-flex;background:transparent;border:1px solid var(--border);color:var(--muted);border-radius:8px;padding:5px 8px;cursor:pointer}.auth__signout:hover{color:var(--err);border-color:var(--err)}.login{max-width:420px;margin:24px auto;text-align:center}.login h1{font-size:24px;font-weight:600;margin:0 0 6px}.login__card{background:var(--surface);border:1px solid var(--border);border-radius:12px;padding:22px;margin-top:18px;display:flex;flex-direction:column;gap:14px}.login__google{justify-content:center}.login__or{color:var(--muted);font-size:13px}.login__email{display:flex;flex-direction:column;gap:10px;text-align:left}.login__email button{justify-content:center}.practice--gate{text-align:center;display:flex;flex-direction:column;align-items:center;gap:12px}.practice--gate p{margin:0;color:var(--muted)}.module__standard{font-size:12.5px;color:var(--muted);margin:-6px 0 14px}.explain__body ul,.explain__body ol{margin:6px 0;padding-left:20px}.explain__body li{font-size:14px;line-height:1.6;color:var(--ink)}.worked__deepdive{display:inline-flex;align-items:center;gap:7px;margin-top:14px;font-size:13.5px;font-weight:600;color:var(--accent);text-decoration:none;border:1px solid var(--accent);border-radius:999px;padding:7px 14px}.worked__deepdive:hover{background:#e1f5ee}.academy{max-width:760px}.academy__kicker{display:inline-flex;align-items:center;gap:6px;font-size:12.5px;font-weight:600;color:var(--accent);margin:0 0 4px}.academy__title{font-size:26px;font-weight:700;margin:0 0 10px;line-height:1.25}.academy__lead{font-size:16px;line-height:1.7;color:var(--ink);margin:0 0 8px}.academy__section{margin-top:22px}.academy__section h2{font-size:18px;font-weight:600;margin:0 0 8px}.academy__section p{font-size:15px;line-height:1.75;color:var(--ink);margin:0 0 8px}.academy__section ul{margin:6px 0;padding-left:20px}.academy__section li{font-size:15px;line-height:1.7}.academy__example{border:1px solid var(--border);border-radius:10px;padding:12px 14px;margin-bottom:12px;background:var(--surface)}.academy__example h3{font-size:15px;font-weight:600;margin:0 0 6px}.academy__summary{background:#e1f5ee;border-radius:12px;padding:14px 16px}.academy__summary h2{margin-top:0}.academy__actions{display:flex;align-items:center;gap:16px;margin-top:26px;flex-wrap:wrap}.academy .button{display:inline-flex;align-items:center;gap:7px;background:var(--accent);color:#fff;text-decoration:none;border-radius:8px;padding:9px 16px;font-size:14px;font-weight:500}.academy__back{color:var(--muted);text-decoration:none;font-size:13.5px}.academy__back:hover{color:var(--accent)}.academy .button{border:none;cursor:pointer}.modal__backdrop{position:fixed;top:0;right:0;bottom:0;left:0;z-index:100;overflow-y:auto;background:#0f171780;display:flex;align-items:flex-start;justify-content:center;padding:6vh 16px}.modal{background:var(--surface);border:1px solid var(--border);border-radius:14px;width:100%;max-width:560px;outline:none;box-shadow:0 20px 50px #00000040}.modal__head{display:flex;align-items:center;justify-content:space-between;padding:14px 18px;border-bottom:1px solid var(--border)}.modal__title{margin:0;font-size:16px;font-weight:600;color:var(--ink)}.modal__close{display:inline-flex;background:transparent;border:none;cursor:pointer;color:var(--muted);padding:4px;border-radius:6px}.modal__close:hover{color:var(--ink)}.modal__body{padding:16px 18px}.modal__body .practice{margin:0}