.shell {
  display: flex;
  flex: 1 1 auto;
  align-items: stretch;
  justify-content: center;
  width: 100%;
  min-height: 100dvh;
  background: #000000; /* 显式黑，桌面端 500px 外圈为黑色 */
}

.content {
  position: relative;
  display: flex;
  flex: 1 1 auto;
  flex-direction: column;
  width: 100%;
  max-width: 500px;
  min-height: 100dvh;
}