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/search.md

@@ -0,0 +1,13 @@
+# 搜索
+
+本站点启用了 VitePress 的本地搜索（Local Search）。
+
+## 如何使用
+
+- 点击右上角的搜索框，输入关键词即可搜索当前语言下的文档内容
+- 桌面端快捷键：`⌘ K`（macOS）/ `Ctrl K`（Windows / Linux）打开搜索
+
+## 小贴士
+
+- 搜不到时先确认语言是否正确（中英文内容分别在 `/` 与 `/zh/`）
+- 搜索对组件名/函数名很友好，建议直接搜 `MarkdownRender`、`enableMermaid`、`enableKatex` 等关键字