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/monorepo-migration.md

@@ -0,0 +1,5 @@
+# Monorepo 迁移指南
+
+此页面为 `monorepo-migration.md` 的中文版本，帮助中文读者了解如何将解析器抽离为 `stream-markdown-parser` 包以及相应的构建与发布流程。
+
+（内容与 `docs/monorepo-migration.md` 相同，已经翻译）