/* ── Free Body Diagram & Force Resolver ──────────────────────────── */
*, *::before, *::after { box-sizing: border-box; margin: 0; padding: 0; }

:root {
  --bg:        #0d1117;
  --surface:   #161b27;
  --surface2:  #1f2535;
  --border:    #2a3050;
  --accent:    #3f8cff;
  --accent-lo: rgba(63,140,255,.22);
  --green:     #3ddc84;
  --red:       #ff5555;
  --gold:      #f5c842;
  --blue:      #42a5f5;
  --text:      #dde3f0;
  --text-dim:  #8b9dc3;
  --font:      'Segoe UI', system-ui, sans-serif;
  --mono:      'Courier New', monospace;
  --radius:    12px;
}

body { background: var(--bg); color: var(--text); font-family: var(--font); min-height: 100vh; padding: 0 16px; }
#app { max-width: 1040px; margin: 0 auto; display: flex; flex-direction: column; gap: 14px; }

header { text-align: center; padding: 4px 0; }
header h1 {
  font-size: 1.8rem; font-weight: 700;
  background: linear-gradient(120deg, #3f8cff 0%, #9ec5ff 100%);
  -webkit-background-clip: text; -webkit-text-fill-color: transparent; background-clip: text;
}
.subtitle { color: var(--text-dim); font-size: 0.85rem; margin-top: 3px; }

/* ── Bars / chrome ── */
.controls-bar {
  display: flex; flex-wrap: wrap; align-items: center; gap: 14px;
  padding: 12px 16px;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
}
.ctrl-group { display: flex; align-items: center; gap: 8px; }
.ctrl-label { font-size: 0.72rem; text-transform: uppercase; letter-spacing: .06em; color: var(--text-dim); white-space: nowrap; }

.pill-tabs { display: flex; background: var(--bg); border-radius: 8px; padding: 3px; gap: 2px; flex-wrap: wrap; }
.pill {
  padding: 7px 14px; border: none; background: transparent; color: var(--text-dim);
  border-radius: 6px; cursor: pointer; font-size: 0.81rem; min-height: 40px;
  transition: background .18s, color .18s, box-shadow .18s;
  touch-action: manipulation; -webkit-tap-highlight-color: transparent;
}
.pill.active { background: var(--accent); color: #fff; box-shadow: 0 0 12px var(--accent-lo); }
.pill:hover:not(.active) { background: var(--surface2); color: var(--text); }

.btn {
  padding: 8px 18px; border: none; border-radius: 8px; cursor: pointer;
  font-size: 0.84rem; font-weight: 600; transition: filter .18s, box-shadow .18s, opacity .18s;
}
.btn:disabled { opacity: 0.38; cursor: not-allowed; }
.btn-primary { background: var(--accent); color: #fff; }
.btn-primary:not(:disabled):hover { filter: brightness(1.12); box-shadow: 0 0 12px var(--accent-lo); }
.btn-ghost { background: var(--surface2); color: var(--text); border: 1px solid var(--border); }
.btn-ghost:hover { background: var(--border); }

.btn-guide {
  margin-left: auto;
  padding: 6px 16px; border: none; border-radius: 8px; cursor: pointer;
  font-size: 0.82rem; font-weight: 700; text-decoration: none;
  background: var(--gold); color: #1a1a2e;
  transition: filter .18s, box-shadow .18s;
}
.btn-guide:hover { filter: brightness(1.1); box-shadow: 0 0 14px rgba(245,200,66,.35); }

#tool-bar #btn-sound { margin-left: auto; }

.rev-select {
  padding: 8px 10px; border-radius: 8px; border: 1px solid var(--border);
  background: var(--bg); color: var(--text); font-size: 0.85rem; cursor: pointer;
}

/* view toggle chips */
.toggle-group { flex-wrap: wrap; gap: 6px; }
.toggle-chip {
  padding: 6px 12px; border-radius: 16px; border: 1px solid var(--border);
  background: var(--bg); color: var(--text-dim); font-size: 0.78rem; cursor: pointer;
  min-height: 36px; transition: all .15s; -webkit-tap-highlight-color: transparent;
}
.toggle-chip:hover { border-color: var(--accent); color: var(--text); }
.toggle-chip.active { background: var(--accent-lo); border-color: var(--accent); color: var(--accent); font-weight: 600; }

/* ── Canvas ── */
.canvas-card {
  width: 100%; background: #0a0e14;
  border: 1px solid var(--border); border-radius: var(--radius);
  padding: 0; position: relative; overflow-x: clip;
}
.canvas-card canvas { display: block; width: 100%; height: 100%; min-height: 320px; touch-action: none; cursor: crosshair; }

.canvas-overlay-btns { position: absolute; bottom: 10px; right: 10px; display: flex; gap: 6px; z-index: 5; }
.canvas-overlay-btn {
  width: 34px; height: 34px; border-radius: 8px;
  background: rgba(22,27,39,0.8); border: 1px solid var(--border);
  color: var(--text-dim); font-size: 0.9rem; cursor: pointer;
  display: flex; align-items: center; justify-content: center;
  transition: background .18s, color .18s, border-color .18s; backdrop-filter: blur(4px);
}
.canvas-overlay-btn:hover { background: var(--accent-lo); border-color: var(--accent); color: var(--accent); }

.ctx-menu {
  position: absolute; z-index: 100; min-width: 168px;
  background: var(--surface); border: 1px solid var(--border); border-radius: 8px;
  padding: 4px 0; box-shadow: 0 8px 24px rgba(0,0,0,.4);
}
.ctx-item {
  display: block; width: 100%; padding: 8px 16px; border: none;
  background: transparent; color: var(--text); font-size: 0.82rem;
  text-align: left; cursor: pointer; transition: background .12s;
}
.ctx-item:hover { background: var(--accent-lo); color: var(--accent); }

/* ── Readout badges ── */
.readout-badges { display: flex; flex-wrap: wrap; gap: 8px; }
.readout-badge {
  display: flex; align-items: center; gap: 7px;
  padding: 9px 14px; background: var(--surface);
  border: 1px solid var(--border); border-radius: var(--radius);
  font-size: 0.78rem; color: var(--text-dim);
}
.readout-badge strong { color: var(--text); font-family: var(--mono); font-size: 0.92rem; font-weight: 700; }
.readout-badge.big { flex: 1; min-width: 170px; }
.readout-badge.big strong { color: var(--accent); font-size: 1.2rem; }
.rb-dot { width: 10px; height: 10px; border-radius: 50%; flex-shrink: 0; }

/* ── Forces panel ── */
.forces-panel {
  padding: 14px 16px;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
}
.forces-head { display: flex; align-items: center; justify-content: space-between; gap: 10px; margin-bottom: 10px; }
.forces-list { display: flex; flex-direction: column; gap: 8px; }
.force-row {
  display: flex; align-items: center; gap: 10px; flex-wrap: wrap;
  padding: 8px 10px; background: var(--bg); border: 1px solid var(--border); border-radius: 8px;
}
.force-swatch { width: 16px; height: 16px; border-radius: 4px; flex-shrink: 0; border: 1px solid rgba(255,255,255,.25); }
.force-name { font-weight: 700; color: var(--text); font-size: 0.85rem; min-width: 26px; }
.force-field { display: flex; align-items: center; gap: 5px; }
.force-field label { font-size: 0.72rem; color: var(--text-dim); }
.force-input {
  width: 78px; padding: 6px 8px; border-radius: 6px; border: 1px solid var(--border);
  background: var(--surface2); color: var(--text); font-size: 0.85rem; font-family: var(--mono);
}
.force-input:focus { outline: 1px solid var(--accent); border-color: var(--accent); }
.force-unit { font-size: 0.72rem; color: var(--text-dim); }
.force-del {
  margin-left: auto; background: none; border: 1px solid var(--border); border-radius: 6px;
  color: var(--text-dim); cursor: pointer; width: 30px; height: 30px; font-size: 1rem;
  transition: all .15s;
}
.force-del:hover { border-color: var(--red); color: var(--red); }
.forces-hint { font-size: 0.75rem; color: var(--text-dim); margin-top: 10px; line-height: 1.5; }

/* ── Explore ── */
.item-selector { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 12px 14px; }
.is-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(150px, 1fr)); gap: 8px; margin-top: 8px; }
.is-card {
  background: var(--bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 10px; cursor: pointer; transition: border-color .15s, background .15s;
}
.is-card:hover { border-color: var(--accent); background: var(--surface2); }
.is-card.active { border-color: var(--accent); background: var(--accent-lo); }
.is-card-name { font-size: 0.82rem; font-weight: 600; color: var(--text); }

.item-info { background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); padding: 18px 20px; line-height: 1.6; }
.item-info h3 { font-size: 1.1rem; color: var(--accent); margin-bottom: 8px; }
.item-info p { font-size: 0.85rem; color: var(--text-dim); margin-bottom: 8px; }
.item-info .formula-box {
  background: var(--bg); border: 1px solid var(--border); border-radius: 8px;
  padding: 10px 14px; font-family: var(--mono); font-size: 0.85rem; color: var(--gold); margin: 8px 0;
}
.item-info .example-box {
  background: rgba(63,140,255,.06); border: 1px solid rgba(63,140,255,.2); border-radius: 8px;
  padding: 10px 14px; margin-top: 10px;
}
.item-info .example-box .step { font-size: 0.82rem; color: var(--text-dim); font-family: var(--mono); }

/* ── Practice / Quiz ── */
.practice-panel, .question-panel {
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 18px 20px;
}
.pp-prompt, .q-prompt { font-size: 0.95rem; line-height: 1.5; margin-bottom: 14px; }
.pp-input-row { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
.pp-input, .q-num-input input {
  width: 130px; padding: 8px 12px; border-radius: 8px; border: 1px solid var(--border);
  background: var(--bg); color: var(--text); font-size: 1rem; font-family: var(--mono);
}
.pp-unit { font-size: 0.9rem; color: var(--accent); font-weight: 600; }
.pp-bottom { margin-top: 12px; }
.feedback { font-weight: 600; font-size: 0.9rem; }
.feedback.correct { color: var(--green); }
.feedback.wrong { color: var(--red); }
.solution-panel {
  margin-top: 12px; padding: 12px 14px; border-radius: 8px;
  background: var(--bg); border: 1px solid var(--border);
  font-size: 0.85rem; line-height: 1.6; color: var(--text-dim);
}
.solution-panel strong { color: var(--text); }
.practice-bar, .quiz-bar {
  display: flex; align-items: center; gap: 14px;
  padding: 10px 16px;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
}
.pbar-score { font-size: 0.85rem; color: var(--accent); font-weight: 600; }

.q-options { display: flex; flex-direction: column; gap: 8px; }
.q-opt {
  padding: 10px 14px; border-radius: 8px; cursor: pointer;
  background: var(--bg); border: 1px solid var(--border);
  font-size: 0.88rem; transition: border-color .15s, background .15s;
}
.q-opt:hover { border-color: var(--accent); background: var(--surface2); }
.q-opt.selected { border-color: var(--accent); background: var(--accent-lo); }
.q-opt.correct { border-color: var(--green); background: rgba(61,220,132,.12); color: var(--green); }
.q-opt.wrong { border-color: var(--red); background: rgba(255,85,85,.12); color: var(--red); }
.qbar-counter { display: flex; align-items: baseline; gap: 4px; }
.qbar-q-label { font-size: 0.7rem; color: var(--text-dim); text-transform: uppercase; }
.qbar-q-num { font-size: 1.2rem; font-weight: 700; color: var(--accent); }
.qbar-q-sep { color: var(--text-dim); }
.qbar-q-total { font-size: 0.9rem; color: var(--text-dim); }
.quiz-result {
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
  padding: 20px; text-align: center;
}
.quiz-result h3 { font-size: 1.3rem; color: var(--accent); margin-bottom: 8px; }
.quiz-result .score { font-size: 2rem; font-weight: 700; color: var(--gold); }
.quiz-result .stars { font-size: 1.6rem; margin: 8px 0; color: var(--gold); }

/* ── SEO article ── */
.seo-article {
  padding: 28px 24px; line-height: 1.75;
  background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius);
}
.seo-article h2 { font-size: 1.3rem; color: var(--text); margin-bottom: 12px; }
.seo-article h3 { font-size: 1.05rem; color: var(--accent); margin: 18px 0 6px; }
.seo-article p { font-size: 0.92rem; color: var(--text-dim); margin-bottom: 10px; }
.seo-article a { color: var(--accent); text-decoration: underline; }
.seo-article a:hover { color: #9ec5ff; }
.seo-article table { width: 100%; border-collapse: collapse; margin: 10px 0 14px; font-size: 0.85rem; }
.seo-article th { text-align: left; padding: 7px 10px; border-bottom: 2px solid var(--border); color: var(--text); font-weight: 600; }
.seo-article td { padding: 6px 10px; border-bottom: 1px solid var(--border); color: var(--text-dim); }
.seo-article strong { color: var(--text); }

.pill:focus-visible, .btn:focus-visible, .btn-guide:focus-visible, .toggle-chip:focus-visible,
.pp-input:focus-visible, .q-opt:focus-visible, .is-card:focus-visible,
.force-input:focus-visible, .rev-select:focus-visible {
  outline: 2px solid var(--accent); outline-offset: 2px;
}

/* ── Responsive ── */
@media (max-width: 768px) {
  header h1 { font-size: 1.5rem; }
  .readout-badge.big strong { font-size: 1.05rem; }
}
@media (max-width: 500px) {
  header h1 { font-size: 1.3rem; }
  .controls-bar { padding: 8px 10px; gap: 8px; }
  .pill { padding: 6px 11px; font-size: 0.75rem; }
  .canvas-card canvas { min-height: 260px; }
  .force-input { width: 64px; }
}

/* ═══ Show-Calculations modal ═══════════════════════════════════ */
.calc-modal { display: none; position: fixed; inset: 0; z-index: 1200; background: rgba(0,0,0,0.72); backdrop-filter: blur(4px); align-items: flex-start; justify-content: center; padding: 24px 16px; overflow-y: auto; }
.calc-modal.active { display: flex; }
.calc-modal-box { background: #141a2e; border: 1px solid var(--border); border-radius: 12px; width: 100%; max-width: 760px; display: flex; flex-direction: column; box-shadow: 0 24px 64px rgba(0,0,0,0.6); max-height: calc(100vh - 48px); overflow: hidden; }
.calc-modal-header { display: flex; align-items: center; justify-content: space-between; padding: 14px 20px; border-bottom: 1px solid var(--border); background: var(--surface2); flex-shrink: 0; }
.calc-modal-title { font-size: 1rem; font-weight: 800; color: var(--text); }
.calc-modal-close { width: 34px; height: 34px; border: 1px solid var(--border); border-radius: 8px; background: var(--bg); color: var(--text-dim); font-size: 1.4rem; line-height: 1; padding: 0; display: flex; align-items: center; justify-content: center; cursor: pointer; transition: all .15s; }
.calc-modal-close:hover { border-color: var(--accent); color: var(--text); }
.calc-modal-body { padding: 16px 20px; overflow-y: auto; display: flex; flex-direction: column; gap: 12px; }
.cs-inputs { background: var(--accent-lo); border: 1px solid var(--accent); border-radius: 8px; padding: 12px 14px; display: flex; flex-direction: column; gap: 8px; }
.cs-badge { display: inline-block; padding: 3px 12px; border-radius: 20px; background: var(--accent-lo); color: var(--accent); font-size: 0.72rem; font-weight: 700; text-transform: uppercase; letter-spacing: .06em; align-self: flex-start; }
.cs-given { display: flex; flex-wrap: wrap; gap: 6px 16px; }
.cs-given span { font-family: var(--mono); font-size: 0.82rem; color: var(--text); }
.cs-si-note { font-size: 0.72rem; color: var(--text-dim); margin: 0; }
.cs-step { background: var(--bg); border: 1px solid var(--border); border-radius: 8px; padding: 12px 14px; display: flex; flex-direction: column; gap: 6px; }
.cs-step-hd { display: flex; align-items: baseline; gap: 8px; flex-wrap: wrap; }
.cs-num { font-size: 0.64rem; font-weight: 800; text-transform: uppercase; letter-spacing: .06em; color: var(--accent); background: var(--accent-lo); padding: 2px 8px; border-radius: 20px; white-space: nowrap; }
.cs-title { font-size: 0.9rem; font-weight: 700; color: var(--text); }
.cs-formula { font-family: var(--mono); font-size: 0.82rem; color: var(--text); background: var(--surface2); border-radius: 6px; padding: 6px 10px; border-left: 3px solid var(--accent); }
.cs-calc { font-family: var(--mono); font-size: 0.8rem; color: var(--text); white-space: pre-wrap; line-height: 1.7; }
.cs-result { font-size: 0.88rem; color: var(--text-dim); margin-top: 2px; }
.cs-result strong { font-family: var(--mono); font-size: 0.95rem; color: var(--green); }

/* ═══ Learning panels (live equations) ═══════════════════════════ */
.learn-panels { padding: 14px; background: var(--surface); border: 1px solid var(--border); border-radius: var(--radius); }
.learn-head { display: flex; align-items: center; justify-content: space-between; gap: 12px; flex-wrap: wrap; padding-bottom: 10px; border-bottom: 1px solid var(--border); margin-bottom: 12px; }
.learn-head-title { font-size: 0.95rem; font-weight: 700; color: var(--text); }
.learn-card { border: 1px solid var(--border); border-radius: 10px; background: var(--bg); overflow: hidden; }
.learn-card[open] { border-color: var(--accent); }
.learn-card > summary { list-style: none; cursor: pointer; padding: 10px 14px; display: flex; align-items: center; gap: 10px; flex-wrap: wrap; font-size: 0.88rem; font-weight: 700; color: var(--text); user-select: none; transition: background .15s; }
.learn-card > summary::-webkit-details-marker { display: none; }
.learn-card > summary::after { content: '\25BE'; margin-left: auto; color: var(--text-dim); font-size: 0.78rem; transition: transform .2s; }
.learn-card[open] > summary::after { transform: rotate(180deg); color: var(--accent); }
.lp-badge { display: inline-flex; align-items: center; justify-content: center; width: 22px; height: 22px; border-radius: 6px; background: var(--accent-lo); font-size: 0.9rem; flex-shrink: 0; color: var(--accent); }
.lp-sub { color: var(--text-dim); font-weight: 500; font-size: 0.78rem; }
.learn-body { padding: 12px 14px 14px; font-size: 0.84rem; color: var(--text-dim); line-height: 1.7; }
.learn-body .eq-line { font-size: 0.95rem; color: var(--text); background: var(--surface2); border-left: 3px solid var(--accent); padding: 8px 10px; border-radius: 6px; margin: 6px 0; overflow-x: auto; }
@media (max-width: 500px) { .lp-sub { display: none; } }