feat(chat): 增强AI回复渲染 — 代码高亮/KaTeX数学/删除线/URL自动链接

新增渲染能力：
1. highlight.js 代码语法高亮 + 复制按钮 (hover显示)
2. KaTeX 数学公式渲染 ($$...$$块级 + $...$行内)
3. 删除线 ~~text~~ → <del>
4. 裸URL自动转可点击链接

依赖: npm install highlight.js katex mermaid

frontend/package.json

@@ -13,8 +13,11 @@
   "license": "ISC",
   "dependencies": {
     "@types/qrcode": "^1.5.6",
+    "highlight.js": "^11.11.1",
     "html2canvas": "^1.4.1",
     "jspdf": "^4.2.1",
+    "katex": "^0.17.0",
+    "mermaid": "^11.16.0",
     "qrcode": "^1.5.4",
     "react": "^18.3.1",
     "react-dom": "^18.3.1",