feat(desktop): embed BoosAPI Desktop client as a subproject

Vendor the rebranded BoosAPI Desktop client (Tauri + Vue) into metapi as
a monorepo subdirectory under desktop/. Source only — node_modules,
dist, Cargo target and the upstream .git history are excluded.

desktop/ keeps its own independent Cargo workspace (desktop/src-tauri
with crates/*); it is not part of the backend boosapi crate, so a
`cargo check` at the metapi root still compiles only the backend. Build
the desktop app from desktop/src-tauri per desktop/README.md.

Note: app icons are still placeholder assets pending the final BoosAPI
logo. The updater signing keys live outside the repo and are not
included.

desktop/libs/markstream-vue/docs/zh/guide/playground.md

@@ -0,0 +1,36 @@
+---
+description: 使用本地和在线 playground 快速复现 markstream-vue 渲染问题、对比框架接入，并验证修复结果。
+---
+
+# Playground（示例演示）
+
+仓库包含一个可交互的 `playground` 演示，用于本地调试和复现渲染问题。
+
+## 在线演示
+
+- Vue 3: https://markstream-vue.simonhe.me/
+- React: https://markstream-react.pages.dev/
+- React migration demo: https://markstream-react.pages.dev/migration-demo
+- Nuxt: https://markstream-nuxt.pages.dev/
+- Vue 2: https://markstream-vue2.pages.dev/
+
+## 本地运行
+
+运行演示：
+
+```bash
+pnpm install
+pnpm play
+# 打开终端中显示的网址（通常为 http://localhost:5173）
+```
+
+在演示中你可以尝试：
+
+- 实时流式 Markdown 输入
+- 渐进式 Mermaid
+- 自定义组件映射
+- Monaco 编辑器集成示例
+
+可直接打开的示例页面：
+
+- `https://markstream-react.pages.dev/migration-demo`：面向 `react-markdown` 用户的 before / after 迁移演示