/* ... (Mobile Responsive CSS to append to style.css) ... */
.mobile-only {
    display: none;
}

/* Sidebar Overlay Mobile */
.sidebar-overlay {
    display: none;
    position: fixed;
    top: 0; left: 0; right: 0; bottom: 0;
    background: rgba(0,0,0,0.5);
    z-index: 99;
}

@media (max-width: 992px) {
    .app-layout {
        grid-template-columns: 300px 1fr;
    }
    .sidebar {
        position: fixed;
        left: -320px;
        top: 0;
        bottom: 0;
        width: 300px;
        z-index: 100;
        background: var(--bg-main);
        transition: left 0.3s ease;
        margin: 0;
        border-radius: 0;
    }
    .sidebar.show {
        left: 0;
    }
    .mobile-only {
        display: inline-flex;
    }
    .sidebar-overlay.show {
        display: block;
    }
}

@media (max-width: 768px) {
    .app-layout {
        grid-template-columns: 1fr;
        margin: 0;
        height: 100vh;
        border-radius: 0;
    }
    .glass-panel {
        border-radius: 0;
        border: none;
    }
    
    /* Logic for showing only one panel at a time */
    .app-layout.show-view #panel-mail-list {
        display: none;
    }
    .app-layout:not(.show-view) #panel-mail-view {
        display: none;
    }
    
    .list-header {
        padding: 15px;
    }
    .read-header {
        padding: 15px;
    }
}