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-nuxt/nuxt.config.ts

@@ -0,0 +1,57 @@
+import { defineNuxtConfig } from 'nuxt/config'
+
+export default defineNuxtConfig({
+  devtools: { enabled: true },
+  app: {
+    head: {
+      title: 'markstream-nuxt Playground | Nuxt demo',
+      meta: [
+        {
+          name: 'description',
+          content: 'markstream-nuxt playground for streaming Markdown, Mermaid, KaTeX, and docs-site integration demos.',
+        },
+      ],
+      link: [
+        { rel: 'icon', type: 'image/svg+xml', href: '/favicon.svg' },
+      ],
+    },
+  },
+  modules: [
+    '@nuxtjs/tailwindcss',
+  ],
+  css: [
+    '@/assets/tailwind.css',
+    'markstream-vue/index.css',
+    'katex/dist/katex.min.css',
+  ],
+  nitro: {
+    preset: 'static',
+    publicAssets: [
+      { dir: 'public' },
+      { dir: '../playground/public', baseURL: '/' },
+    ],
+  },
+  vite: {
+    optimizeDeps: {
+      include: [
+        '@antv/infographic',
+        '@floating-ui/dom',
+        '@terrastruct/d2',
+        'katex',
+        'katex/dist/contrib/mhchem',
+        'mermaid',
+        'stream-markdown',
+        'vue-i18n',
+      ],
+      exclude: ['monaco-editor', 'stream-monaco', 'markstream-vue'],
+    },
+    worker: {
+      format: 'es',
+    },
+  },
+  runtimeConfig: {
+    public: {
+      demoTitle: 'Nuxt + Vue Renderer Markdown',
+    },
+  },
+})