:root {
    --bg: #0b1020;
    --panel: #121a33;
    --accent: #6ea8fe;
    --text: #e6e9f2;
    --muted: #9aa3b2;
    --danger: #ff6b6b;
    user-select: none;
}

@media (max-width: 360px) {
    body {
        font-size: 12px;
    }
}

@media (min-width: 361px) and (max-width: 720px) {
    body {
        font-size: 14px;
    }
}

@media (min-width: 721px) and (max-width: 1080px) {
    body {
        font-size: 16px;
    }
}

@media (min-width: 1081px) and (max-width: 1440px) {
    body {
        font-size: 18px;
    }
}

@media (min-width: 1441px) {
    body {
        font-size: 20px;
    }
}

* {
    box-sizing: border-box;
    user-select: none;
}

html, body {
    height: 100%;
    user-select: none;
}

body {
    margin: 0;
    font: 16px/1.2 system-ui, -apple-system, Segoe UI, Roboto, Helvetica, Arial, "Apple Color Emoji", "Segoe UI Emoji";
    color: var(--text);
    background: radial-gradient(1200px 800px at 70% -20%, #1f2a53 0%, var(--bg) 55%);
    display: grid;
    place-items: center;
    padding: 24px;
    user-select: none;
}

.calc {
    width: min(92vw, 380px);
    background: linear-gradient(180deg, #182447, var(--panel));
    border-radius: 24px;
    box-shadow: 0 20px 60px rgba(0,0,0,.45), inset 0 1px 0 rgba(255,255,255,.06);
    overflow: hidden;
    outline: none;
    user-select: none;
}

.display {
    padding: 24px 20px 10px;
    min-height: 116px;
    display: grid;
    gap: 6px;
    background: linear-gradient(180deg, rgba(255,255,255,.03), rgba(255,255,255,.0));
    user-select: none;
}

.prev {
    color: var(--muted);
    font-size: 14px;
    letter-spacing: .3px;
    min-height: 20px;
    white-space: nowrap;
    overflow: hidden;
    text-overflow: ellipsis;
    user-select: none;
}

.curr {
    font-weight: 700;
    font-size: clamp(28px, 8vw, 40px);
    letter-spacing: .4px;
    text-align: right;
    padding-left: 8px;
    word-break: break-all;
    user-select: none;
}

.keys {
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    padding: 14px;
    background: rgba(0,0,0,.06);
    user-select: none;
}

button.key {
    appearance: none;
    border: none;
    border-radius: 16px;
    padding: 16px 14px;
    font-size: 18px;
    font-weight: 600;
    color: var(--text);
    background: linear-gradient(180deg, #213058, #192446);
    box-shadow: 0 6px 12px rgba(0,0,0,.25), inset 0 1px 0 rgba(255,255,255,.05);
    cursor: pointer;
    transition: transform .06s ease, box-shadow .12s ease, background .12s ease;
    will-change: transform;
    user-select: none;
}

button.key:active {
    transform: translateY(1px) scale(.995);
    user-select: none;
}

button.key:focus-visible {
    outline: 2px solid var(--accent);
    outline-offset: 2px;
    box-shadow: 0 0 0 4px rgba(110,168,254,.25), 0 6px 12px rgba(0,0,0,.25);
    user-select: none;
}

.key.op {
    background: linear-gradient(180deg, #2b3a6e, #24315b);
    user-select: none;
}

.key.equals {
    background: linear-gradient(180deg, #4d6ed6, #3a56b1);
    user-select: none;
}

.key.ac {
    background: linear-gradient(180deg, #7b2431, #551923);
    user-select: none;
}

.key.span-2 {
    grid-column: span 2;
    user-select: none;
}

.sr-only {
    position: absolute;
    width: 1px;
    height: 1px;
    padding: 0;
    margin: -1px;
    overflow: hidden;
    clip: rect(0,0,0,0);
    border: 0;
    user-select: none;
}

.back-link {
    margin-top: 20px;
    text-align: center;
    user-select: none;
}

.back-link a {
    display: inline-block;
    padding: 12px 20px;
    border-radius: 12px;
    background: linear-gradient(180deg, #2b3a6e, #24315b);
    color: var(--text);
    text-decoration: none;
    font-weight: 600;
    transition: background .2s ease;
    user-select: none;
}

.back-link a:hover {
    background: linear-gradient(180deg, #3a56b1, #2b3a6e);
    user-select: none;
}