```css
/* CSS for Signup Modal Flow - Dark Theme */

.signup-modal {
    width: 100vw;
    height: 100vh;
    max-width: none;
    /* Wide for split view */
    background: #1e1e1e;
    color: #e5e7eb;
    border-radius: 0;
    border: none;
    box-shadow: none;
    overflow: hidden;
    position: fixed;
    top: 0;
    left: 0;
    display: flex;
    flex-direction: column;
    animation: fadeIn 0.3s ease-out;
}

@keyframes scaleIn {
    from {
        transform: scale(0.95);
        opacity: 0;
    }

    to {
        transform: scale(1);
        opacity: 1;
    }
}

/* Header */
.signup-header {
    height: 56px;
    border-bottom: 1px solid rgba(255, 255, 255, 0.08);
    display: flex;
    align-items: center;
    justify-content: space-between;
    padding: 0 1.5rem;
    background: rgba(255, 255, 255, 0.02);
}

.signup-title {
    display: flex;
    align-items: center;
    gap: 0.75rem;
    font-weight: 600;
    font-size: 1rem;
}

.signup-icon {
    width: 24px;
    height: 24px;
    border-radius: 6px;
}

.close-signup {
    background: none;
    border: none;
    color: #9ca3af;
    cursor: pointer;
    font-size: 1.25rem;
}

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

/* Progress Bar */
.signup-progress-track {
    height: 4px;
    background: #374151;
    width: 100%;
}

.signup-progress-bar {
    height: 100%;
    background: #111827;
    /* Dark filled bar */
    transition: width 0.5s ease;
}

/* Content Body */
.signup-content {
    flex: 1;
    position: relative;
    display: flex;
    flex-direction: column;
    align-items: center;
    justify-content: center;
    /* Crucial: Vertical Center */
    padding-bottom: 5vh;
    /* Visual balance */
}

.signup-view {
    display: none;
    width: 100%;
    /* Constrain the inner content width so it doesn't stretch to screen edges */
    max-width: 1000px;
    margin: 0 auto;
    padding: 2rem;
    animation: fadeIn 0.4s;
    justify-content: center;
    align-items: center;
}

.signup-view.active {
    display: flex;
}

/* View Layouts */
.split-view {
    flex-direction: column;
    gap: 2rem;
}

@media (min-width: 768px) {
    .split-view {
        flex-direction: row;
        gap: 0;
        align-items: center;
        /* Center items vertically */
    }

    .view-left,
    .view-right {
        flex: 1;
        padding: 2rem;
        display: flex;
        flex-direction: column;
        justify-content: center;
        align-items: center;
        /* Center content like spinners */
    }

    .view-left {
        align-items: flex-start;
        /* Left text aligns left usually */
        text-align: left;
    }

    /* Special override for the loading screen left text to be centered if desired, 
       but typically 'Preparing...' looks good centered or left. Let's force center for 'centered-col' class users */
    .view-left.centered-col {
        align-items: center;
        text-align: center;
    }

    .view-right {
        background: #18181b;
        /* Slightly darker right panel */
        border-left: 1px solid rgba(255, 255, 255, 0.05);
    }
}

.single-col {
    justify-content: center;
    align-items: center;
}

.centered-col {
    flex-direction: column;
    justify-content: center;
    align-items: center;
    text-align: center;
    gap: 1rem;
}

/* Spinner */
.spinner-large {
    width: 48px;
    height: 48px;
    border: 3px solid rgba(255, 255, 255, 0.1);
    border-top-color: white;
    border-radius: 50%;
    animation: spin 1s linear infinite;
    margin-bottom: 1rem;
}

@keyframes spin {
    to {
        transform: rotate(360deg);
    }
}

/* Forms */
.form-section,
.form-columns {
    width: 100%;
    max-width: 480px;
}

.form-group {
    margin-bottom: 1rem;
}

.form-row {
    display: flex;
    gap: 1rem;
}

.form-row .form-group {
    flex: 1;
}

.form-label {
    display: block;
    margin-bottom: 0.4rem;
    font-size: 0.85rem;
    font-weight: 500;
    color: #d1d5db;
}

.uppercase {
    text-transform: uppercase;
    font-size: 0.75rem;
    letter-spacing: 0.05em;
}

.form-input {
    width: 100%;
    padding: 0.75rem;
    background: #27272a;
    border: 1px solid #3f3f46;
    border-radius: 6px;
    color: white;
    font-size: 0.95rem;
}

.form-input:focus {
    outline: none;
    border-color: #6366f1;
    ring: 2px solid rgba(99, 102, 241, 0.2);
}

.text-muted {
    color: #9ca3af;
}

.small {
    font-size: 0.85rem;
}

.btn-row {
    display: flex;
    justify-content: space-between;
    margin-top: 2rem;
    gap: 1rem;
}

.btn-primary {
    background: #111827;
    /* Black/Dark button */
    color: white;
    padding: 0.6rem 1.2rem;
    border-radius: 6px;
    border: 1px solid rgba(255, 255, 255, 0.1);
    font-weight: 500;
    cursor: pointer;
}

.btn-primary:hover {
    background: #000;
}

.btn-primary:disabled {
    opacity: 0.5;
    cursor: not-allowed;
}

.btn-secondary {
    background: transparent;
    color: #d1d5db;
    padding: 0.6rem 1.2rem;
    border-radius: 6px;
    border: 1px solid #3f3f46;
    cursor: pointer;
}

.btn-secondary:hover {
    background: rgba(255, 255, 255, 0.05);
}

/* Mock UI Panel */
.mock-mailbox-card {
    background: #1e1e1e;
    /* Match modal bg or slightly lighter */
    border-radius: 16px;
    width: 100%;
    max-width: 380px;
    /* Wider */
    height: 500px;
    /* Taller fixed height */
    overflow: hidden;
    box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.5), 0 2px 4px -1px rgba(0, 0, 0, 0.3);
    border: 1px solid rgba(255, 255, 255, 0.1);
    display: flex;
    flex-direction: column;
}

.mock-header {
    background: #27272a;
    padding: 16px 20px;
    display: flex;
    justify-content: space-between;
    align-items: center;
    border-bottom: 1px solid rgba(255, 255, 255, 0.05);
}

.mock-logo {
    font-size: 0.9rem;
    font-weight: 700;
    color: #f3f4f6;
}

.mock-btn {
    background: #000;
    color: white;
    padding: 4px 10px;
    font-size: 0.75rem;
    border-radius: 6px;
    border: 1px solid rgba(255, 255, 255, 0.2);
}

.mock-body-grid {
    flex: 1;
    /* Fill remaining space */
    display: grid;
    grid-template-columns: 100px 1fr;
    background: #18181b;
    /* Darker body bg */
}

.mock-sidebar {
    background: #1f1f22;
    border-right: 1px solid rgba(255, 255, 255, 0.05);
    padding: 16px;
    display: flex;
    flex-direction: column;
    gap: 12px;
}

.mock-list {
    padding: 16px;
    display: flex;
    flex-direction: column;
    gap: 16px;
}

/* Make lines brighter/more visible */
.mock-line {
    height: 8px;
    background: #3f3f46;
    border-radius: 4px;
}

.w-60 {
    width: 60%;
}

.w-80 {
    width: 80%;
}

.w-40 {
    width: 40%;
}

.w-100 {
    width: 100%;
}

.w-70 {
    width: 70%;
}

.w-90 {
    width: 90%;
}

.mock-footer {
    padding: 2rem;
    background: #1e1e1e;
    text-align: center;
    border-top: 1px solid rgba(255, 255, 255, 0.05);
    min-height: 140px;
    /* Ensure content area is stable */
}

.tiny-label {
    font-size: 0.7rem;
    letter-spacing: 0.1em;
    color: #9ca3af;
    margin-bottom: 0.75rem;
    text-transform: uppercase;
    font-weight: 600;
}

.mock-footer h4 {
    margin: 0 0 0.5rem 0;
    color: white;
    font-size: 1.25rem;
}

.mock-footer p {
    color: #9ca3af;
    font-size: 0.95rem;
    margin: 0 0 1.5rem 0;
    min-height: 3em;
    line-height: 1.5;
}

.feature-dots {
    display: flex;
    justify-content: center;
    gap: 8px;
}

.dot {
    width: 32px;
    height: 4px;
    background: #3f3f46;
    border-radius: 2px;
    transition: background 0.3s;
}

.dot.active {
    background: #e5e7eb;
}

/* Validation Rules Block */
.validation-rules {
    margin-top: 1.5rem;
    padding-top: 1rem;
    border-top: 1px solid rgba(255, 255, 255, 0.08);
    font-size: 0.75rem;
    color: #6b7280;
    line-height: 1.4;
    text-align: center;
}

/* Terms Modal */
.modal-backdrop {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.7);
    backdrop-filter: blur(4px);
    z-index: 200;
    display: flex;
    align-items: center;
    justify-content: center;
    padding: 1rem;
}

.terms-card {
    background: #1e1e1e;
    width: 100%;
    max-width: 600px;
    max-height: 80vh;
    border-radius: 12px;
    border: 1px solid #3f3f46;
    display: flex;
    flex-direction: column;
}

.terms-header {
    padding: 1rem 1.5rem;
    border-bottom: 1px solid #3f3f46;
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.terms-body {
    padding: 1.5rem;
    overflow-y: auto;
    font-size: 0.9rem;
    line-height: 1.6;
    color: #d1d5db;
}

.terms-footer {
    padding: 1rem 1.5rem;
    border-top: 1px solid #3f3f46;
    display: flex;
    align-items: center;
    justify-content: space-between;
}

.close-btn {
    background: none;
    border: none;
    font-size: 1.25rem;
    color: #9ca3af;
    cursor: pointer;
}

/* Errors */
.form-error {
    color: #ef4444;
    font-size: 0.85rem;
    margin-top: 0.5rem;
    min-height: 1.2em;
}

.checkbox-row {
    display: flex;
    gap: 0.5rem;
    align-items: flex-start;
    margin-bottom: 0.75rem;
    font-size: 0.9rem;
    color: #d1d5db;
    cursor: pointer;
}

.checkbox-row a {
    color: #6366f1;
    text-decoration: none;
}