ui: 抽离流式消息渲染组件并增强富文本显示；

将助手消息渲染从页面内联逻辑拆分为独立组件，接入 Markdown/GFM、代码高亮与公式渲染，提升流式输出在表格、代码块等主流格式下的展示能力。

Made-with: Cursor

src/pages/index.tsx

@@ -13,6 +13,7 @@ import {
 } from "@ant-design/icons";
 import { Button, Collapse, Drawer, Input, Layout, Typography } from "antd";
 import { isAbortLikeError, streamQwenChat, type ChatMessagePayload } from "../api/qwenChat";
+import StreamMessage from "../components/StreamMessage";
 
 const { Header, Sider, Content } = Layout;
 const MOBILE_WIDTH = 750;
@@ -376,11 +377,7 @@ export default function HomePage() {
                           ]}
                         />
                       )}
-                      {item.role === "assistant" && (
-                        <div className="rounded-2xl border border-[var(--ds-border)] bg-neutral-50/80 px-4 py-2.5 text-[15px] leading-relaxed text-neutral-800">
-                          {item.content}
-                        </div>
-                      )}
+                      {item.role === "assistant" && <StreamMessage content={item.content} />}
                       {item.role === "user" && item.content}
                     </div>
                   </div>