/* --- base + tokens --- */
:root {
  --bg-deep: #0c0f14;
  --bg-glow: #141a24;
  --glass: rgba(255, 255, 255, 0.06);
  --glass-border: rgba(255, 255, 255, 0.12);
  --glass-highlight: rgba(255, 255, 255, 0.18);
  --text: #e8ecf1;
  --text-muted: #8b95a8;
  --accent: #5eead4;
  --accent-dim: rgba(94, 234, 212, 0.15);
  --danger: #f87171;
  --danger-soft: rgba(248, 113, 113, 0.2);
  --key-num: rgba(255, 255, 255, 0.08);
  --key-op: rgba(94, 234, 212, 0.12);
  --shadow: 0 24px 48px rgba(0, 0, 0, 0.45);
  --radius: 18px;
  --radius-sm: 12px;
  --font: "DM Sans", system-ui, sans-serif;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

html {
  -webkit-tap-highlight-color: transparent;
}

body {
  margin: 0;
  min-height: 100dvh;
  font-family: var(--font);
  color: var(--text);
  background: var(--bg-deep);
  display: flex;
  align-items: center;
  justify-content: center;
  padding: 1.25rem;
}

/* soft gradient orbs behind everything */
.page-bg {
  position: fixed;
  inset: 0;
  z-index: -1;
  background:
    radial-gradient(ellipse 80% 60% at 15% 20%, rgba(94, 234, 212, 0.12), transparent 50%),
    radial-gradient(ellipse 70% 50% at 85% 75%, rgba(99, 102, 241, 0.1), transparent 45%),
    linear-gradient(165deg, var(--bg-deep), var(--bg-glow));
}

.wrap {
  width: 100%;
  max-width: 380px;
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

.brand {
  text-align: center;
}

.brand h1 {
  margin: 0;
  font-size: clamp(1.35rem, 4vw, 1.6rem);
  font-weight: 700;
  letter-spacing: -0.02em;
}

.tagline {
  margin: 0.35rem 0 0;
  font-size: 0.875rem;
  color: var(--text-muted);
}

/* glass shell */
.calculator {
  background: var(--glass);
  backdrop-filter: blur(20px);
  -webkit-backdrop-filter: blur(20px);
  border: 1px solid var(--glass-border);
  border-radius: var(--radius);
  padding: 1.25rem;
  box-shadow: var(--shadow);
}

.display-area {
  background: rgba(0, 0, 0, 0.25);
  border-radius: var(--radius-sm);
  padding: 0.85rem 1rem;
  margin-bottom: 1rem;
  min-height: 5.5rem;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  align-items: flex-end;
  border: 1px solid rgba(255, 255, 255, 0.06);
}

.display-sub {
  font-size: 0.8rem;
  color: var(--text-muted);
  min-height: 1.2em;
  word-break: break-all;
  text-align: right;
  width: 100%;
}

.display-main {
  font-size: clamp(1.75rem, 7vw, 2.25rem);
  font-weight: 600;
  letter-spacing: -0.03em;
  line-height: 1.2;
  word-break: break-all;
  text-align: right;
  width: 100%;
  display: block;
}

.display-main.is-error {
  color: var(--danger);
  font-size: clamp(1.1rem, 5vw, 1.5rem);
}

/* button grid */
.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.55rem;
}

.key {
  font-family: inherit;
  font-size: 1.05rem;
  font-weight: 600;
  border: 1px solid var(--glass-border);
  border-radius: var(--radius-sm);
  padding: 0.95rem 0.5rem;
  cursor: pointer;
  color: var(--text);
  transition:
    transform 0.12s ease,
    background 0.15s ease,
    border-color 0.15s ease,
    box-shadow 0.15s ease;
}

.key:focus-visible {
  outline: 2px solid var(--accent);
  outline-offset: 2px;
}

.key-num {
  background: var(--key-num);
}

.key-op {
  background: var(--key-op);
  color: var(--accent);
}

.key-fn {
  background: rgba(255, 255, 255, 0.05);
  color: var(--text-muted);
  font-size: 0.85rem;
}

.key-equals {
  grid-column: span 2;
  background: linear-gradient(145deg, rgba(94, 234, 212, 0.35), rgba(94, 234, 212, 0.15));
  border-color: var(--glass-highlight);
  color: #042f2e;
}

.key-zero {
  grid-column: span 1;
}

/* C gets extra width — top row felt cramped with four equal keys */
.key-wide {
  grid-column: span 2;
}

/* hover / active — gives that tactile feedback */
.key:hover {
  border-color: var(--glass-highlight);
  box-shadow: 0 4px 16px rgba(0, 0, 0, 0.2);
}

.key:active {
  transform: scale(0.96);
}

.key-equals:hover {
  box-shadow: 0 6px 20px var(--accent-dim);
}

.hint {
  margin: 0;
  font-size: 0.75rem;
  color: var(--text-muted);
  text-align: center;
  line-height: 1.45;
  padding: 0 0.5rem;
}

/* tablet / desktop: slightly roomier */
@media (min-width: 480px) {
  .keys {
    gap: 0.65rem;
  }

  .key {
    padding: 1.05rem 0.5rem;
    font-size: 1.1rem;
  }

  .calculator {
    padding: 1.5rem;
  }
}

/* very small phones */
@media (max-width: 320px) {
  .keys {
    gap: 0.4rem;
  }

  .key {
    padding: 0.8rem 0.35rem;
    font-size: 0.95rem;
  }
}