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/why.md

@@ -0,0 +1,21 @@
+# Why use markstream-vue
+
+- Progressive Mermaid: diagrams render incrementally so users see results earlier.
+- Streaming diff code blocks: show diffs as they arrive for instant feedback.
+- Built for scale: optimized DOM updates and memory use for very large documents.
+
+This library is especially suited for real-time, AI-driven, and large-document scenarios where a conventional, static Markdown pipeline would cause lag or break UX.
+
+Try this — test the renderer quickly in a browser component:
+
+```vue
+<script setup>
+import MarkdownRender from 'markstream-vue'
+
+const md = '# Why test\n\nStreaming works well in large docs'
+</script>
+
+<template>
+  <MarkdownRender :content="md" />
+</template>
+```