/* Shell layout — #main-nav, #app, responsive breakpoints (UI-01, UI-08) */

#main-nav {
    position: sticky;
    top: 0;
    z-index: 100;
    display: flex;
    align-items: center;
    justify-content: space-between;
    height: 56px;
    padding: 0 var(--space-4);
    background: var(--color-bg-subtle);
    border-bottom: 1px solid var(--color-border);
}

#main-nav .nav-brand {
    font-size: var(--font-size-base);
    font-weight: var(--font-weight-semibold);
    color: var(--color-text);
}

#main-nav .nav-links {
    display: flex;
    gap: var(--space-6);
    list-style: none;
    margin: 0;
    padding: 0;
}

#main-nav .nav-links a {
    display: inline-flex;
    align-items: center;
    min-height: 44px; /* touch target */
    padding: 0 var(--space-2);
    color: var(--color-text);
    font-weight: var(--font-weight-normal);
    border-bottom: 2px solid transparent;
}

#main-nav .nav-links a.is-active {
    color: var(--color-text);
    font-weight: var(--font-weight-semibold);
    border-bottom-color: var(--color-accent);
}

#app {
    display: block;
    min-height: calc(100vh - 56px);
    padding: var(--space-4);
}

/* Tablet (≥640px) */
@media (min-width: 640px) {
    #main-nav { padding: 0 var(--space-6); }
    #app     { padding: var(--space-6); }
}

/* Desktop (≥1024px) */
@media (min-width: 1024px) {
    #app {
        max-width: 1280px;
        margin: 0 auto;
        padding: var(--space-6);
    }
}