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/guide/playground.md

@@ -0,0 +1,55 @@
+---
+description: Explore local and online markstream-vue playgrounds to reproduce rendering issues, compare frameworks, and verify fixes quickly.
+---
+
+# Playground
+
+This repository includes a playable demo under `/playground` that helps you test features locally and reproduce rendering issues.
+
+## Online Playgrounds
+
+- 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/
+
+## Running Locally
+
+How to run the playground locally:
+
+```bash
+pnpm install
+pnpm play
+# Open the dev server shown in terminal (usually http://localhost:5173)
+```
+
+The playground demonstrates:
+
+- Live streaming Markdown input with progressive Mermaid diagrams
+- Custom components mapping and `setCustomComponents`
+- Monaco streaming integration and code block examples
+
+Example pages (open from the playground):
+
+- `https://markstream-vue.simonhe.me/test` — shareable repro page for stress-testing streaming features (hosted demo)
+- `https://markstream-react.pages.dev/migration-demo` — hosted before/after migration demo for `react-markdown` users
+- `/markdown` — Markdown vs static rendering comparison
+
+![Playground demo](/screenshots/playground-demo.svg)
+
+If you want to add examples to the playground for documentation pages, keep them minimal and focused (the `playground/src/pages` folder is a good place to add pages).
+
+Quick try — add a simple playground page by creating `playground/src/pages/quick-test.vue` and pasting a small test like:
+
+```vue twoslash
+<script setup lang="ts">
+import MarkdownRender from 'markstream-vue'
+
+const md = '# Quick playground test\n\nThis is a demo.'
+</script>
+
+<template>
+  <MarkdownRender :content="md" />
+</template>
+```