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/playground-vue2/src/main.ts

@@ -0,0 +1,22 @@
+import { setKaTeXWorker, setMermaidWorker, VueRendererMarkdown } from 'markstream-vue2'
+import KatexWorker from 'markstream-vue2/workers/katexRenderer.worker?worker&inline'
+import MermaidWorker from 'markstream-vue2/workers/mermaidParser.worker?worker&inline'
+import Vue from 'vue'
+import App from './App.vue'
+import 'markstream-vue2/index.css'
+import 'katex/dist/katex.min.css'
+import 'monaco-editor/min/vs/editor/editor.main.css'
+import '@unocss/reset/tailwind.css'
+import './styles/main.css'
+import './app.css'
+
+Vue.use(VueRendererMarkdown)
+
+if (typeof window !== 'undefined') {
+  setKaTeXWorker(new KatexWorker())
+  setMermaidWorker(new MermaidWorker())
+}
+
+new Vue({
+  render: h => h(App),
+}).$mount('#app')