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/zh/guide/compared.md

@@ -0,0 +1,8 @@
+# 对比
+
+将 `markstream-vue` 与常用选项进行对比：
+
+- 与纯静态渲染相比，`markstream-vue` 支持流式渲染与逐步更新，适用于实时编辑场景。
+- 与大型 Markdown-to-HTML 框架相比，目标是更小的内存占用与更快的首次渲染（以避免大量 DOM 操作）。
+
+（更多细节参见英文版 `/guide/compared` 或中文版 `/zh/guide/compared`）