html, body {
  margin: 0;
  height: 100%;
  overflow: hidden;
  background: #0e1620;
  display: flex;
  align-items: center;
  justify-content: center;
  font-family: system-ui, -apple-system, "Segoe UI", "Microsoft YaHei", sans-serif;
}

/* 画布逻辑尺寸固定 960x640，按窗口等比缩放 */
#cv {
  width: min(100vw, 150vh);
  height: auto;
  display: block;
  border-radius: 12px;
  background: #eef2f5;
  box-shadow: 0 24px 70px rgba(0, 0, 0, .55);
}