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/compared.md

@@ -0,0 +1,25 @@
+# Compared to traditional Markdown renderers
+
+Traditional Markdown renderers typically convert a finished Markdown string into a static HTML tree. This library is designed for streaming and interactive workflows and therefore provides capabilities you won't find in a classic renderer:
+
+- Streaming-first rendering: render partial or incrementally-updated Markdown content without re-parsing the whole document each time. This enables live previews for AI outputs or editors that emit tokens progressively.
+- Streaming-aware code blocks and "code-jump" UX: large code blocks are updated incrementally and the renderer can maintain cursor/selection context and fine-grained edits.
+- Built-in diff/code-stream components: show diffs as they arrive (line-by-line or token-by-token) with minimal reflow.
+- Progressive diagrams and editors: Mermaid and Monaco-based previews update progressively.
+- Flexible code block rendering: pick Monaco for interactive editing or Shiki for display-only highlighting.
+
+Quick try — render streaming content and static fallback side-by-side:
+
+```vue
+<script setup>
+const streaming = '# Streaming example\n\nThis appears progressively'
+const staticMd = '# Static example\n\nRendered once'
+</script>
+
+<template>
+  <div>
+    <MarkdownRender :content="streaming" />
+    <MarkdownRender :content="staticMd" />
+  </div>
+</template>
+```