:root {
    --bg-color: #0b0d14;
    --text-color: #ffffff;
    --glass-bg: rgba(16, 20, 32, 0.70);
    --glass-border: rgba(255, 255, 255, 0.08);
    --primary-glow: rgba(99, 102, 241, 0.3);
    
    /* Charge colors */
    --positive-color: #ff453a;
    --positive-glow: rgba(255, 69, 58, 0.3);
    --negative-color: #0a84ff;
    --negative-glow: rgba(10, 132, 255, 0.3);

    /* Accent */
    --accent: #818cf8;
    --accent-glow: rgba(129, 140, 248, 0.25);

    /* Selection */
    --select-color: #fbbf24;
    --select-glow: rgba(251, 191, 36, 0.3);

    /* Danger */
    --danger-color: #ef4444;
}

* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
    user-select: none;
    -webkit-tap-highlight-color: transparent;
}

body {
    font-family: 'Inter', sans-serif;
    background-color: var(--bg-color);
    color: var(--text-color);
    overflow: hidden;
    width: 100vw;
    height: 100vh;
    height: -webkit-fill-available;
}

#simulationCanvas {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    z-index: 1;
    cursor: crosshair;
    touch-action: none;
}

/* === Welcome Overlay === */
.welcome-overlay {
    position: absolute;
    top: 0; left: 0; width: 100%; height: 100%;
    z-index: 5;
    display: flex;
    justify-content: center;
    align-items: center;
    pointer-events: none;
    transition: opacity 0.6s ease;
}

.welcome-overlay.fade-out {
    opacity: 0;
}

.welcome-content {
    text-align: center;
    animation: welcomePulse 3s ease-in-out infinite;
    padding: 0 1rem;
}

.welcome-icon {
    font-size: 3.5rem;
    margin-bottom: 0.8rem;
    filter: drop-shadow(0 0 20px rgba(129, 140, 248, 0.5));
}

.welcome-content h2 {
    font-size: 1.6rem;
    font-weight: 600;
    color: #e0e7ff;
    margin-bottom: 0.5rem;
    letter-spacing: -0.3px;
}

.welcome-content p {
    font-size: 0.95rem;
    color: rgba(255,255,255,0.7);
    max-width: 340px;
    line-height: 1.6;
    margin-bottom: 1.2rem;
}

.welcome-hints {
    display: flex;
    gap: 1.2rem;
    justify-content: center;
    font-size: 0.78rem;
    color: rgba(255,255,255,0.3);
}

@keyframes welcomePulse {
    0%, 100% { opacity: 0.7; transform: translateY(0); }
    50% { opacity: 1; transform: translateY(-4px); }
}

/* === UI Overlay === */
.ui-overlay {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    pointer-events: none;
    z-index: 10;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
    padding: 1.5rem;
}

/* === Glass Panels === */
.glass-panel {
    background: var(--glass-bg);
    backdrop-filter: blur(14px);
    -webkit-backdrop-filter: blur(14px);
    border: 1px solid var(--glass-border);
    border-radius: 14px;
    padding: 1.4rem;
    width: 320px;
    pointer-events: auto;
    box-shadow:
        0 4px 24px 0 rgba(0, 0, 0, 0.5),
        inset 0 1px 0 rgba(255, 255, 255, 0.04);
    position: relative;
}

.glass-panel-small {
    background: var(--glass-bg);
    backdrop-filter: blur(14px);
    -webkit-backdrop-filter: blur(14px);
    border: 1px solid var(--glass-border);
    border-radius: 10px;
    padding: 1rem;
    width: 310px;
    pointer-events: auto;
    box-shadow:
        0 2px 12px rgba(0, 0, 0, 0.4),
        inset 0 1px 0 rgba(255, 255, 255, 0.03);
}

/* === HUD === */
.hud h3 {
    font-size: 0.9rem;
    margin-bottom: 0.7rem;
    color: #e2e8f0;
    border-bottom: 1px solid rgba(255, 255, 255, 0.1);
    padding-bottom: 0.4rem;
    letter-spacing: 0.3px;
}

.reading {
    font-size: 0.82rem;
    color: #a5b4fc;
    margin-bottom: 0.35rem;
    font-family: 'SF Mono', 'Cascadia Code', 'Consolas', monospace;
}
.reading strong {
    color: #e0e7ff;
    font-family: 'Inter', sans-serif;
    font-weight: 500;
}

.reading-divider {
    height: 1px;
    background: rgba(255,255,255,0.06);
    margin: 0.6rem 0;
}

.selected-info h3 {
    font-size: 0.85rem;
    margin-bottom: 0.5rem;
    color: var(--select-color);
    border-bottom: 1px solid rgba(251, 191, 36, 0.15);
    padding-bottom: 0.3rem;
}

.hidden {
    display: none !important;
}

/* === Header === */
h1 {
    font-size: 1.6rem;
    font-weight: 600;
    margin-bottom: 0.4rem;
    color: #e0e7ff;
    letter-spacing: -0.3px;
    animation: titleGlow 4s ease-in-out infinite;
}

@keyframes titleGlow {
    0%, 100% { text-shadow: 0 0 8px rgba(129, 140, 248, 0.0); }
    50% { text-shadow: 0 0 18px rgba(129, 140, 248, 0.35), 0 0 40px rgba(129, 140, 248, 0.1); }
}

p {
    font-size: 0.9rem;
    color: rgba(255, 255, 255, 0.75);
    margin-bottom: 1rem;
    line-height: 1.5;
}

.stats {
    font-size: 0.85rem;
    color: #cbd5e1;
    margin-bottom: 1.2rem;
    padding: 0.5rem 0.8rem;
    background: rgba(0, 0, 0, 0.3);
    border-radius: 6px;
    display: inline-block;
    font-family: 'SF Mono', 'Cascadia Code', 'Consolas', monospace;
}

/* === Button group === */
.btn-group {
    display: flex;
    gap: 0.5rem;
}

.btn-primary {
    background: linear-gradient(135deg, #4f46e5, #818cf8);
    color: white;
    border: none;
    padding: 0.75rem 1.4rem;
    border-radius: 8px;
    font-size: 0.95rem;
    font-weight: 600;
    cursor: pointer;
    flex: 1;
    transition: all 0.3s cubic-bezier(0.175, 0.885, 0.32, 1.275);
    box-shadow: 0 4px 15px var(--accent-glow);
    position: relative;
    overflow: hidden;
}

.btn-primary:hover {
    background: linear-gradient(135deg, #6366f1, #a5b4fc);
    transform: translateY(-2px) scale(1.02);
    box-shadow: 0 8px 25px rgba(129, 140, 248, 0.4);
}

.btn-primary:active {
    transform: translateY(1px) scale(0.98);
}

.btn-danger {
    background: rgba(239, 68, 68, 0.12);
    color: var(--danger-color);
    border: 1px solid rgba(239, 68, 68, 0.25);
    padding: 0.75rem 1rem;
    border-radius: 8px;
    font-size: 0.85rem;
    font-weight: 600;
    cursor: pointer;
    transition: all 0.25s ease;
    white-space: nowrap;
}

.btn-danger:hover {
    background: rgba(239, 68, 68, 0.2);
    border-color: rgba(239, 68, 68, 0.5);
    transform: translateY(-1px);
}

.btn-danger:active {
    transform: translateY(0);
}

.btn-icon {
    position: absolute;
    top: 0.7rem;
    right: 0.7rem;
    width: 28px;
    height: 28px;
    border-radius: 50%;
    background: rgba(255,255,255,0.06);
    border: 1px solid rgba(255,255,255,0.1);
    color: rgba(255,255,255,0.5);
    font-size: 0.85rem;
    font-weight: 700;
    cursor: pointer;
    display: flex;
    align-items: center;
    justify-content: center;
    transition: all 0.2s;
}

.btn-icon:hover {
    background: rgba(255,255,255,0.12);
    color: white;
}

/* === Shortcuts Panel === */
.shortcuts-panel {
    position: absolute;
    top: 1.5rem;
    left: 340px;
    width: 240px;
    z-index: 20;
    pointer-events: auto;
    animation: fadeSlideIn 0.2s ease;
}

.shortcuts-panel h3 {
    font-size: 0.85rem;
    color: #c7d2fe;
    margin-bottom: 0.6rem;
    border-bottom: 1px solid rgba(255,255,255,0.06);
    padding-bottom: 0.3rem;
}

.shortcut-row {
    display: flex;
    align-items: center;
    justify-content: space-between;
    font-size: 0.78rem;
    color: rgba(255,255,255,0.5);
    margin-bottom: 0.35rem;
}

kbd {
    background: rgba(255,255,255,0.08);
    border: 1px solid rgba(255,255,255,0.15);
    border-radius: 4px;
    padding: 0.15rem 0.4rem;
    font-family: 'SF Mono', 'Cascadia Code', 'Consolas', monospace;
    font-size: 0.72rem;
    color: #c7d2fe;
}

@keyframes fadeSlideIn {
    from { opacity: 0; transform: translateX(-8px); }
    to { opacity: 1; transform: translateX(0); }
}

/* === Toggle Panel (bottom right) === */
.toggle-panel {
    position: fixed;
    bottom: 1.5rem;
    right: 1.5rem;
    width: 220px;
    z-index: 10;
    pointer-events: auto;
}

.toggle-panel h3 {
    font-size: 0.85rem;
    color: #c7d2fe;
    margin-bottom: 0.7rem;
    border-bottom: 1px solid rgba(255,255,255,0.06);
    padding-bottom: 0.3rem;
    letter-spacing: 0.3px;
}

.toggle-row {
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0.35rem 0;
    cursor: pointer;
    font-size: 0.82rem;
    color: rgba(255,255,255,0.6);
    transition: color 0.2s;
}

.toggle-row:hover {
    color: rgba(255,255,255,0.85);
}

.toggle-row input[type="checkbox"] {
    display: none;
}

.toggle-switch {
    position: relative;
    width: 36px;
    height: 20px;
    background: rgba(255,255,255,0.1);
    border-radius: 10px;
    transition: background 0.25s ease;
    flex-shrink: 0;
}

.toggle-switch::after {
    content: '';
    position: absolute;
    top: 2px;
    left: 2px;
    width: 16px;
    height: 16px;
    border-radius: 50%;
    background: rgba(255,255,255,0.4);
    transition: all 0.25s ease;
}

input[type="checkbox"]:checked + .toggle-switch {
    background: var(--accent);
    box-shadow: 0 0 10px var(--accent-glow);
}

input[type="checkbox"]:checked + .toggle-switch::after {
    left: 18px;
    background: white;
}

/* === Modal === */
.modal-backdrop {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    z-index: 100;
    display: flex;
    justify-content: center;
    align-items: center;
    opacity: 1;
    pointer-events: auto;
    animation: fadeIn 0.2s ease;
}

@keyframes fadeIn {
    from { opacity: 0; }
    to { opacity: 1; }
}

.modal-backdrop.hidden {
    display: none;
}

.glass-modal {
    background: #181a28;
    border: 1px solid rgba(255, 255, 255, 0.08);
    border-radius: 16px;
    padding: 2rem;
    width: 400px;
    max-width: 92vw;
    max-height: 85vh;
    overflow-y: auto;
    -webkit-overflow-scrolling: touch;
    box-shadow: 0 16px 48px rgba(0, 0, 0, 0.65);
    position: relative;
    animation: modalSlideIn 0.25s ease;
}

@keyframes modalSlideIn {
    from { opacity: 0; transform: translateY(12px) scale(0.97); }
    to { opacity: 1; transform: translateY(0) scale(1); }
}

.close-btn {
    position: absolute;
    top: 0.5rem;
    right: 0.5rem;
    background: none;
    border: none;
    color: rgba(255, 255, 255, 0.4);
    font-size: 1.5rem;
    cursor: pointer;
    transition: color 0.2s;
    width: 44px;
    height: 44px;
    display: flex;
    align-items: center;
    justify-content: center;
}

.close-btn:hover {
    color: white;
}

.glass-modal h2 {
    margin-bottom: 1.5rem;
    font-size: 1.15rem;
    font-weight: 600;
    border-bottom: 1px solid rgba(255,255,255,0.06);
    padding-bottom: 0.5rem;
    color: #e0e7ff;
}

.form-group {
    margin-bottom: 1.4rem;
}

.form-group label {
    display: block;
    font-size: 0.85rem;
    color: #a5b4fc;
    margin-bottom: 0.5rem;
    font-weight: 500;
}

.modern-select {
    width: 100%;
    padding: 0.7rem 0.9rem;
    border-radius: 8px;
    background: rgba(0,0,0,0.25);
    border: 1px solid rgba(255,255,255,0.15);
    color: white;
    font-family: inherit;
    font-size: 16px; /* Prevents iOS auto-zoom on focus */
    outline: none;
    transition: border-color 0.2s;
}

.modern-select:focus {
    border-color: var(--accent);
}

.modern-select option {
    background: #181a28;
    color: white;
}

.slider-container {
    display: flex;
    align-items: center;
    gap: 1rem;
}

input[type=range] {
    appearance: none;
    -webkit-appearance: none;
    flex: 1;
    height: 6px;
    background: rgba(255, 255, 255, 0.12);
    border-radius: 3px;
    outline: none;
}

input[type=range]::-webkit-slider-thumb {
    -webkit-appearance: none;
    appearance: none;
    width: 22px;
    height: 22px;
    border-radius: 50%;
    background: var(--accent);
    cursor: pointer;
    box-shadow: 0 0 8px var(--accent-glow);
    transition: transform 0.15s;
}

input[type=range]::-webkit-slider-thumb:hover {
    transform: scale(1.15);
}

#magnitudeVal, #dimensionVal {
    font-variant-numeric: tabular-nums;
    font-weight: 600;
    min-width: 52px;
    font-size: 0.85rem;
    color: #c7d2fe;
}

.sign-selector {
    display: flex;
    gap: 0.5rem;
}

.sign-selector input[type="radio"] {
    display: none;
}

.sign-btn {
    flex: 1;
    text-align: center;
    padding: 0.7rem;
    border-radius: 8px;
    cursor: pointer;
    font-weight: 600;
    font-size: 0.85rem;
    border: 1px solid rgba(255,255,255,0.08);
    background: rgba(0,0,0,0.2);
    color: #888;
    transition: all 0.2s ease;
}

#signPositive:checked + .positive-btn {
    background: rgba(255, 69, 58, 0.12);
    border-color: var(--positive-color);
    color: var(--positive-color);
    box-shadow: 0 0 12px rgba(255, 69, 58, 0.15);
}

#signNegative:checked + .negative-btn {
    background: rgba(10, 132, 255, 0.12);
    border-color: var(--negative-color);
    color: var(--negative-color);
    box-shadow: 0 0 12px rgba(10, 132, 255, 0.15);
}

.form-actions {
    margin-top: 0.5rem;
}

.btn-submit {
    background: linear-gradient(135deg, #e0e7ff, #ffffff);
    color: #1a1c29;
    border: none;
    padding: 0.85rem;
    border-radius: 8px;
    font-size: 0.95rem;
    font-weight: 600;
    cursor: pointer;
    width: 100%;
    transition: all 0.2s ease;
    box-shadow: 0 2px 8px rgba(255,255,255,0.1);
    min-height: 48px;
}

.btn-submit:hover {
    background: #ffffff;
    transform: translateY(-1px);
    box-shadow: 0 4px 16px rgba(255,255,255,0.15);
}

.btn-submit:active {
    transform: translateY(0);
}

/* ========== RESPONSIVE: MOBILE ========== */
@media (max-width: 768px) {
    .ui-overlay {
        padding: 0.5rem;
    }

    /* Header panel: full width, compact */
    .glass-panel {
        width: calc(100% - 1rem);
        max-width: none;
        padding: 0.8rem;
        border-radius: 10px;
    }
    
    h1 { font-size: 1.1rem; margin-bottom: 0.2rem; }
    
    .glass-panel p {
        font-size: 0.75rem;
        margin-bottom: 0.5rem;
        display: none; /* Hide verbose instructions on mobile to save space */
    }
    
    .stats {
        font-size: 0.7rem;
        margin-bottom: 0.6rem;
        padding: 0.3rem 0.5rem;
    }
    
    .btn-group {
        gap: 0.4rem;
    }
    
    .btn-primary {
        padding: 0.6rem 0.8rem;
        font-size: 0.82rem;
        min-height: 44px;
    }
    
    .btn-danger {
        padding: 0.6rem 0.7rem;
        font-size: 0.78rem;
        min-height: 44px;
    }

    .btn-icon {
        width: 44px;
        height: 44px;
    }

    /* HUD: compact, positioned at bottom-left */
    .hud {
        position: fixed;
        bottom: 0.5rem;
        left: 0.5rem;
        width: auto;
        max-width: 52%;
        padding: 0.5rem;
        z-index: 15;
    }
    .glass-panel-small {
        width: auto;
    }
    .hud h3 {
        font-size: 0.72rem;
        margin-bottom: 0.3rem;
        padding-bottom: 0.2rem;
    }
    .hud .reading {
        font-size: 0.65rem;
        margin-bottom: 0.1rem;
    }
    .hud .reading strong {
        font-size: 0.65rem;
    }
    
    /* Toggle panel: compact, bottom-right */
    .toggle-panel {
        width: auto;
        max-width: 42%;
        bottom: 0.5rem;
        right: 0.5rem;
        padding: 0.5rem;
    }
    .toggle-panel h3 {
        font-size: 0.72rem;
        margin-bottom: 0.3rem;
        padding-bottom: 0.2rem;
    }
    .toggle-row {
        font-size: 0.7rem;
        padding: 0.2rem 0;
    }
    .toggle-switch {
        width: 28px;
        height: 16px;
    }
    .toggle-switch::after {
        width: 12px;
        height: 12px;
    }
    input[type="checkbox"]:checked + .toggle-switch::after {
        left: 14px;
    }
    
    /* Shortcuts panel: hide on mobile */
    .shortcuts-panel {
        display: none;
    }
    
    /* Welcome overlay mobile */
    .welcome-icon { font-size: 2.5rem; }
    .welcome-content h2 { font-size: 1.2rem; }
    .welcome-content p { font-size: 0.82rem; max-width: 280px; }
    .welcome-hints {
        flex-direction: column;
        gap: 0.3rem;
        font-size: 0.7rem;
    }
    
    /* Modal on mobile */
    .glass-modal {
        width: 94vw;
        max-width: none;
        padding: 1.2rem;
        border-radius: 12px;
        max-height: 80vh;
    }
    .glass-modal h2 { font-size: 1rem; }
    .form-group { margin-bottom: 1rem; }

    /* Larger range slider thumb for touch */
    input[type=range]::-webkit-slider-thumb {
        width: 28px;
        height: 28px;
    }
    input[type=range] {
        height: 8px;
    }
}

@media (max-width: 380px) {
    .glass-panel { padding: 0.5rem; }
    h1 { font-size: 0.95rem; }
    .btn-primary { font-size: 0.78rem; padding: 0.5rem 0.6rem; }
    .btn-danger { font-size: 0.72rem; padding: 0.5rem; }
    .hud { max-width: 48%; }
    .toggle-panel { max-width: 44%; }
}