* {
  box-sizing: border-box;
}

body {
  margin: 0;
  font-family: Arial, "Microsoft YaHei", sans-serif;
  background: #f5f6f8;
  color: #222;
}

.app {
  display: flex;
  height: 100vh;
}

.sidebar {
  width: 280px;
  background: #ffffff;
  border-right: 1px solid #ddd;
  padding: 18px;
}

.sidebar h2 {
  margin: 0 0 18px;
  font-size: 20px;
}

#newChatBtn {
  width: 100%;
  padding: 12px;
  border: none;
  border-radius: 8px;
  background: #2563eb;
  color: white;
  font-size: 15px;
  cursor: pointer;
}

.section-title {
  margin: 20px 0 10px;
  color: #666;
  font-size: 14px;
}

.chat-item {
  padding: 10px;
  margin-bottom: 8px;
  background: #f1f3f5;
  border-radius: 8px;
  cursor: pointer;
}

.chat-item.active {
  background: #dbeafe;
}

.main {
  flex: 1;
  display: flex;
  flex-direction: column;
}

header {
  padding: 18px 24px;
  background: #ffffff;
  border-bottom: 1px solid #ddd;
}

header h1 {
  margin: 0;
  font-size: 22px;
}

header p {
  margin: 6px 0 0;
  color: #777;
}

.messages {
  flex: 1;
  padding: 24px;
  overflow-y: auto;
}

.message {
  max-width: 760px;
  padding: 12px 14px;
  margin-bottom: 14px;
  border-radius: 10px;
  line-height: 1.6;
  white-space: pre-wrap;
}

.message.user {
  margin-left: auto;
  background: #2563eb;
  color: white;
}

.message.assistant {
  margin-right: auto;
  background: #ffffff;
  border: 1px solid #ddd;
}

footer {
  display: flex;
  gap: 12px;
  padding: 18px 24px;
  background: #ffffff;
  border-top: 1px solid #ddd;
}

textarea {
  flex: 1;
  height: 54px;
  resize: none;
  padding: 12px;
  font-size: 15px;
  border: 1px solid #ccc;
  border-radius: 8px;
}

#sendBtn {
  width: 90px;
  border: none;
  border-radius: 8px;
  background: #16a34a;
  color: white;
  font-size: 15px;
  cursor: pointer;
}