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/features.md

@@ -0,0 +1,25 @@
+# Features
+
+- Progressive Mermaid: diagrams render incrementally
+- Progressive D2: diagrams render incrementally with source fallback
+- Streaming-first rendering: handle tokenized and partial markdown
+- Monaco streaming integration: efficient updates for large code blocks
+- Streaming diff code blocks: show diffs as they are generated
+- Flexible code rendering: Monaco or Shiki
+- Pluggable parse hooks: pre- and post-transform tokens
+- Full Markdown support (tables, math, optional Emoji, task checkboxes, code blocks). Emoji is provided as an optional plugin and is not enabled by default — enable it via `getMarkdown` options or the component `customMarkdownIt` prop.
+- Lubu features: math rendering via KaTeX, i18n support
+
+Quick try — render a Markdown string with streaming-friendly features:
+
+```vue
+<script setup lang="ts">
+import MarkdownRender from 'markstream-vue'
+
+const md = '# Streamed Features\n\n```mermaid\ngraph LR\nA-->B\n```'
+</script>
+
+<template>
+  <MarkdownRender :content="md" />
+</template>
+```