feat(chat): 消息列表回到底部与贴底滚动优化

- 非底部时显示悬浮回到底部按钮，点击平滑滚底并隐藏
- 仅在已处于底部附近时随新消息自动贴底，避免打断上翻阅读
- 按钮使用原生圆钮并相对输入区 max-w-4xl 定位，避免 AntD 按钮宽条问题
- 同步 lucide-react 依赖及流式代码块、全局样式等小改动

Made-with: Cursor

src/components/StreamMessage.tsx

@@ -1,6 +1,6 @@
 import { useState } from "react";
-import { CheckOutlined, CopyOutlined } from "@ant-design/icons";
 import { Button, message } from "antd";
+import { Check, Copy } from "lucide-react";
 import ReactMarkdown from "react-markdown";
 import remarkGfm from "remark-gfm";
 import remarkMath from "remark-math";
@@ -47,7 +47,7 @@ function MarkdownPreBlock(props: { children: ReactNode }) {
         <Button
           type="text"
           size="small"
-          icon={copied ? <CheckOutlined /> : <CopyOutlined />}
+          icon={copied ? <Check size={16} /> : <Copy size={16} />}
           className="text-neutral-600 hover:bg-black/5! hover:text-neutral-800!"
           onClick={() => {
             void onCopy();