chore(deps): declare mermaid dependency

补上 b0fdeab9 引入 mermaid 图表渲染（roundtable 产物预览）时漏提交的
依赖声明，否则全新 pnpm install 后构建会因找不到 mermaid 失败。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend-web/package.json

@@ -74,6 +74,7 @@
     "hast": "^1.0.0",
     "katex": "^0.16.28",
     "lucide-react": "^0.562.0",
+    "mermaid": "^11.15.0",
     "motion": "^12.26.2",
     "nanoid": "^5.1.6",
     "next-themes": "^0.4.6",