:root {
    --bg: #f7f8fa;
    --panel: #ffffff;
    --ink: #1f2933;
    --muted: #637083;
    --line: #d8dee8;
    --accent: #2563eb;
    --accent-dark: #1d4ed8;
    --ok: #0f766e;
}

* {
    box-sizing: border-box;
}

body {
    margin: 0;
    background: var(--bg);
    color: var(--ink);
    font-family: system-ui, -apple-system, BlinkMacSystemFont, "Segoe UI", sans-serif;
    line-height: 1.5;
}

a {
    color: var(--accent);
    text-decoration: none;
}

.topbar {
    align-items: center;
    background: var(--panel);
    border-bottom: 1px solid var(--line);
    display: flex;
    gap: 24px;
    justify-content: space-between;
    padding: 14px 24px;
}

.brand {
    color: var(--ink);
    font-weight: 700;
}

nav {
    display: flex;
    flex-wrap: wrap;
    gap: 14px;
}

.shell {
    margin: 0 auto;
    max-width: 1040px;
    padding: 28px 20px 56px;
}

.layout-wide {
    max-width: 1280px;
}

.grid,
.form-grid {
    display: grid;
    gap: 16px;
    grid-template-columns: repeat(auto-fit, minmax(260px, 1fr));
}

.card,
.panel {
    background: var(--panel);
    border: 1px solid var(--line);
    border-radius: 8px;
    padding: 18px;
}

.narrow {
    margin: 0 auto;
    max-width: 520px;
}

.muted {
    color: var(--muted);
}

.button,
button,
input[type="submit"] {
    background: var(--accent);
    border: 0;
    border-radius: 6px;
    color: #fff;
    cursor: pointer;
    display: inline-flex;
    font: inherit;
    font-weight: 650;
    gap: 8px;
    padding: 9px 13px;
}

.button.secondary,
button.secondary {
    background: #e8eef8;
    color: var(--ink);
}

.button:hover,
button:hover,
input[type="submit"]:hover {
    background: var(--accent-dark);
}

button:disabled {
    cursor: default;
    opacity: 0.45;
}

label {
    display: block;
    font-weight: 650;
    margin: 14px 0 6px;
}

label.inline {
    align-items: center;
    display: inline-flex;
    gap: 8px;
    margin: 0;
}

.setting-check {
    margin-top: 14px;
}

input,
select,
textarea {
    border: 1px solid var(--line);
    border-radius: 6px;
    font: inherit;
    padding: 10px;
    width: 100%;
}

.inline input {
    width: auto;
}

textarea {
    min-height: 360px;
}

.notice {
    background: #e6f7f4;
    border: 1px solid #b9e4dc;
    border-radius: 8px;
    color: #105d55;
    margin-bottom: 18px;
    padding: 12px 14px;
}

.list {
    display: grid;
    gap: 10px;
}

.row {
    border: 1px solid var(--line);
    border-radius: 8px;
    color: var(--ink);
    display: grid;
    gap: 4px;
    padding: 13px;
}

.row span {
    color: var(--muted);
}

.lesson-frame {
    display: grid;
    gap: 18px;
    grid-template-columns: minmax(0, 1fr) 280px;
}

.chat-panel {
    background: #eef2f7;
    border: 1px solid var(--line);
    border-radius: 8px;
    min-height: 620px;
    padding: 18px;
}

.lesson-stream {
    display: flex;
    flex-direction: column;
    gap: 14px;
}

.lesson-block {
    background: var(--panel);
    border: 1px solid var(--line);
    border-radius: 8px;
    box-shadow: 0 1px 3px rgba(15, 23, 42, 0.06);
    max-width: 820px;
    opacity: 1;
    padding: 18px;
}

.lesson-block.pending {
    display: none;
}

.lesson-block.active {
    border-color: #93b4f7;
}

.lesson-block.streaming {
    border-color: #93b4f7;
}

.lesson-block.checkpoint {
    border-left: 4px solid var(--ok);
}

.stream-cursor {
    border-right: 2px solid var(--accent);
    display: inline-block;
    height: 1em;
    margin-left: 2px;
    transform: translateY(2px);
    width: 1px;
    animation: cursor-blink 1s steps(2, start) infinite;
}

@keyframes cursor-blink {
    50% {
        opacity: 0;
    }
}

pre {
    background: #172033;
    border-radius: 6px;
    color: #f8fafc;
    overflow: auto;
    padding: 14px;
}

.lesson-controls {
    align-items: center;
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
    margin-top: 16px;
}

.stat {
    border-bottom: 1px solid var(--line);
    padding: 10px 0;
}

@media (max-width: 880px) {
    .topbar,
    .lesson-frame {
        display: block;
    }

    nav {
        margin-top: 10px;
    }
}