:root {
  color-scheme: light;
  --spring-ink: #222824;
  --spring-muted: #687069;
  --spring-line: #cfd3cc;
  --spring-line-strong: #9fa79f;
  --spring-accent: #245b49;
  --spring-accent-soft: #e2ebe5;
  --spring-orange: #8b5b2b;
  --spring-orange-soft: #f3e5d5;
  --spring-danger: #9b2f2f;
  --spring-surface: #fbfaf6;
  --spring-paper: #f5f4ef;
}

*, *::before, *::after { box-sizing: border-box; }
button, input, select { font: inherit; }
a { color: inherit; }
p { overflow-wrap: anywhere; }
:focus-visible { outline: 3px solid var(--spring-accent); outline-offset: 3px; }

h2 { margin: 0; font-size: clamp(1.2rem, 2vw, 1.48rem); line-height: 1.35; }
h3 { margin: 0; font-size: 1rem; line-height: 1.4; }

.tool-shell {
  display: grid;
  grid-template-columns: minmax(0, .96fr) minmax(0, 1.04fr);
  border: 1px solid var(--spring-line-strong);
  background: var(--spring-surface);
}

.calculator-panel,
.visual-panel { min-width: 0; padding: 24px; }
.calculator-panel { border-right: 1px solid var(--spring-line); }

.section-heading { min-width: 0; }
.section-kicker {
  margin: 0 0 5px;
  color: var(--spring-accent);
  font-size: .78rem;
  font-weight: 780;
  letter-spacing: .08em;
  text-transform: uppercase;
}

.values-fieldset {
  margin: 17px 0 0;
  padding: 0;
  border: 0;
}
.values-fieldset > legend { margin-bottom: 9px; font-weight: 760; }
.input-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 14px 12px; }
.input-row { min-width: 0; }
.input-row label { display: block; margin-bottom: 5px; font-weight: 720; }
.input-wrap {
  display: grid;
  grid-template-columns: minmax(0, 1fr) minmax(78px, .4fr);
  gap: 6px;
}
.plain-input,
.unit-select {
  width: 100%;
  min-width: 0;
  min-height: 44px;
  border: 1px solid var(--spring-line-strong);
  border-radius: 2px;
  padding: 9px 10px;
  color: var(--spring-ink);
  background: var(--spring-surface);
}
.plain-input:focus,
.unit-select:focus { border-color: var(--spring-accent); }
.plain-input[aria-invalid="true"] {
  border-color: var(--spring-danger);
  background: #fff8f8;
}
.unit-select { cursor: pointer; }
.field-help,
.field-error,
.input-note { margin: 6px 0 0; color: var(--spring-muted); font-size: .79rem; line-height: 1.5; }
.field-error { min-height: 1.2em; color: var(--spring-danger); font-weight: 650; }
.input-note { margin-top: 14px; }

.form-error {
  margin: 17px 0 0;
  padding: 10px 12px;
  border: 1px solid var(--spring-danger);
  background: #fff3f3;
  color: var(--spring-danger);
  font-weight: 680;
}
.form-actions { display: flex; flex-wrap: wrap; gap: 10px; margin-top: 17px; }
.button {
  min-height: 44px;
  padding: 9px 15px;
  border: 1px solid var(--spring-accent);
  border-radius: 2px;
  cursor: pointer;
  font-weight: 750;
}
.button-primary { background: var(--spring-accent); color: #fff; }
.button-secondary { background: transparent; color: var(--spring-accent); }
.button:hover { filter: brightness(.96); }

.result-box {
  margin-top: 20px;
  padding: 18px;
  border: 1px solid var(--spring-line);
  background: var(--spring-paper);
}
.result-box[data-state="ready"] { border-color: var(--spring-accent); background: #f3f8f4; }
.result-box[data-state="error"] { border-color: var(--spring-danger); background: #fff8f8; }
.result-heading-row,
.visual-heading-row { display: flex; align-items: flex-start; justify-content: space-between; gap: 12px; }
.result-label { margin: 0 0 4px; color: var(--spring-muted); font-size: .78rem; font-weight: 720; letter-spacing: .06em; text-transform: uppercase; }
.result-heading-row h2 { overflow-wrap: anywhere; }
.status-badge {
  flex: 0 0 auto;
  padding: 4px 8px;
  border: 1px solid var(--spring-line-strong);
  color: var(--spring-muted);
  font-size: .72rem;
  font-weight: 750;
  line-height: 1.35;
  white-space: nowrap;
}
.status-badge[data-state="ready"] { border-color: var(--spring-accent); background: var(--spring-accent-soft); color: var(--spring-accent); }
.status-badge[data-state="error"] { border-color: var(--spring-danger); background: #fff0f0; color: var(--spring-danger); }
.result-value {
  display: block;
  margin: 13px 0 15px;
  color: var(--spring-accent);
  font-size: clamp(1.55rem, 4vw, 2.25rem);
  font-weight: 760;
  line-height: 1.2;
  overflow-wrap: anywhere;
}
.result-box[data-state="error"] .result-value { color: var(--spring-danger); }
.metrics-grid { display: grid; grid-template-columns: repeat(2, minmax(0, 1fr)); gap: 8px; }
.metric-card {
  min-width: 0;
  padding: 10px;
  border: 1px solid var(--spring-line);
  background: var(--spring-surface);
}
.metric-card span { display: block; color: var(--spring-muted); font-size: .76rem; line-height: 1.4; }
.metric-card strong { display: block; margin-top: 3px; color: var(--spring-ink); font-size: 1rem; overflow-wrap: anywhere; }
.metric-wide { grid-column: 1 / -1; }
.result-details {
  display: grid;
  grid-template-columns: repeat(2, minmax(0, 1fr));
  gap: 8px 14px;
  margin: 14px 0 0;
  padding-top: 13px;
  border-top: 1px solid var(--spring-line);
}
.result-details[hidden] { display: none; }
.result-details > div { min-width: 0; }
.result-details dt { color: var(--spring-muted); font-size: .75rem; }
.result-details dd { margin: 2px 0 0; overflow-wrap: anywhere; }
.detail-wide { grid-column: 1 / -1; }
.result-note { margin: 13px 0 0; color: var(--spring-muted); font-size: .82rem; }

.visual-panel { background: #f8f8f4; }
.visual-metrics {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 8px;
  margin: 18px 0 13px;
}
.visual-metrics > div { min-width: 0; padding: 9px 10px; border: 1px solid var(--spring-line); background: var(--spring-surface); }
.visual-metrics dt { color: var(--spring-muted); font-size: .74rem; }
.visual-metrics dd { margin: 3px 0 0; font-weight: 720; overflow-wrap: anywhere; }
.visual-stage { min-width: 0; border: 1px solid var(--spring-line); background: var(--spring-surface); }
.spring-figure { margin: 0; }
.spring-svg { display: block; width: 100%; height: auto; min-height: 240px; }
.spring-wall { fill: #707971; }
.wall-hatch { fill: none; stroke: #f5f4ef; stroke-width: 2; }
.natural-reference { stroke: var(--spring-line-strong); stroke-width: 2; stroke-dasharray: 5 5; }
.natural-tick { stroke: var(--spring-line-strong); stroke-width: 2; }
.svg-label { fill: var(--spring-muted); font-size: 14px; }
.wall-label { font-size: 13px; font-weight: 720; }
.svg-value { fill: var(--spring-ink); font-size: 13px; }
.spring-coil { fill: none; stroke: var(--spring-accent); stroke-width: 4; stroke-linecap: round; stroke-linejoin: round; }
.spring-mass { fill: var(--spring-accent-soft); stroke: var(--spring-accent); stroke-width: 2; }
.mass-label { fill: var(--spring-accent); font-size: 13px; font-weight: 760; }
.displacement-line { stroke: var(--spring-orange); stroke-width: 2; }
.displacement-label { fill: var(--spring-orange); font-size: 14px; font-weight: 740; }
.force-line { stroke: var(--spring-accent); stroke-width: 3; }
.force-label { fill: var(--spring-accent); font-size: 14px; font-weight: 740; }
.state-label { fill: var(--spring-ink); font-size: 15px; font-weight: 740; }
.energy-track { fill: #e1e4df; }
.energy-fill { fill: var(--spring-orange); }
.energy-label { font-size: 12px; }
.energy-value { font-weight: 700; }
.visual-caption { margin: 0; padding: 10px 13px 13px; color: var(--spring-muted); font-size: .78rem; line-height: 1.5; }
.visual-description { margin: 13px 0 0; color: var(--spring-ink); font-size: .84rem; }
.scale-note { margin: 7px 0 0; color: var(--spring-muted); font-size: .78rem; }

.content-section {
  margin-top: 28px;
  padding-top: 26px;
  border-top: 1px solid var(--spring-line);
}
.steps-list,
.notes-list { margin: 13px 0 0; padding-left: 1.3rem; }
.steps-list li,
.notes-list li { margin: 6px 0; overflow-wrap: anywhere; }
.formula-list { display: grid; grid-template-columns: repeat(3, minmax(0, 1fr)); gap: 10px; margin-top: 15px; }
.formula-list article {
  min-width: 0;
  padding: 13px;
  border: 1px solid var(--spring-line);
  background: var(--spring-surface);
}
.formula-list strong { display: block; color: var(--spring-accent); font-size: 1.14rem; }
.formula-list span { display: block; margin-top: 6px; color: var(--spring-muted); font-size: .82rem; line-height: 1.5; }
.example-box {
  margin-top: 14px;
  padding: 13px 15px;
  border-left: 4px solid var(--spring-accent);
  background: var(--spring-accent-soft);
}
.example-box p { margin: 6px 0 0; }
.notes-section { padding-bottom: 8px; }

@media (max-width: 900px) {
  .tool-shell { grid-template-columns: minmax(0, 1fr); }
  .calculator-panel { border-right: 0; border-bottom: 1px solid var(--spring-line); }
}

@media (max-width: 620px) {
  .calculator-panel, .visual-panel { padding: 18px; }
  .input-grid,
  .metrics-grid,
  .result-details,
  .visual-metrics,
  .formula-list { grid-template-columns: minmax(0, 1fr); }
  .metric-wide, .detail-wide { grid-column: auto; }
  .result-heading-row, .visual-heading-row { display: block; }
  .result-heading-row .status-badge, .visual-heading-row .status-badge { display: inline-block; margin-top: 9px; }
  .spring-svg { min-height: 200px; }
}

@media (max-width: 430px) {
  .calculator-panel, .visual-panel { padding: 15px; }
  .result-box { padding: 14px; }
  .form-actions { display: grid; grid-template-columns: minmax(0, 1fr); }
  .button { width: 100%; }
  .input-wrap { grid-template-columns: minmax(0, 1fr) minmax(72px, .42fr); }
  .svg-label { font-size: 12px; }
  .state-label, .force-label, .displacement-label { font-size: 12px; }
}

@media (prefers-reduced-motion: reduce) {
  html { scroll-behavior: auto; }
}

.rf-figure {margin:18px 0 0;} #rf-svg {display:block;width:100%;height:auto;} #rf-drawing text {font-size:14px;fill:#222824;} #result-values p {padding:8px 0;margin:0;border-bottom:1px solid #cfd3cc;overflow-wrap:anywhere;} .input-grid {margin-top:16px;} .visual-stage {margin-top:18px;}