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/.vitepress/theme/index.ts

@@ -0,0 +1,24 @@
+import type { EnhanceAppContext } from 'vitepress'
+import TwoslashFloatingVue from '@shikijs/vitepress-twoslash/client'
+import MarkdownRender from 'markstream-vue'
+import Theme from 'vitepress/theme'
+import GitHubStarBadge from './GitHubStarBadge.vue'
+import Layout from './Layout.vue'
+import SupportQRCodes from './SupportQRCodes.vue'
+import '@shikijs/vitepress-twoslash/style.css'
+import 'markstream-vue/index.css'
+import './style.css'
+
+export default {
+  extends: Theme,
+  Layout,
+  enhanceApp({ app }: EnhanceAppContext) {
+    app.use(TwoslashFloatingVue)
+    app.component('GitHubStarBadge', GitHubStarBadge)
+    app.component('MarkdownRender', MarkdownRender)
+    app.component('SupportQRCodes', SupportQRCodes)
+  },
+}
+
+// Export useDark for use in VitePress markdown files and components
+export { useDark } from './composables/useDark'