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/math.md

@@ -0,0 +1,42 @@
+# 数学公式（KaTeX）
+
+`markstream-vue` 在检测到 `katex` 时使用 KaTeX 渲染数学。KaTeX 为可选 peer 依赖，需单独安装。
+
+安装示例：
+
+```bash
+pnpm add katex
+```
+
+在入口文件导入样式：
+
+```ts
+import 'katex/dist/katex.min.css'
+```
+
+## CDN 用法（无 bundler）
+
+如果你通过 CDN `<script>` 引入 KaTeX（即 `window.katex` 存在），库会优先使用该全局对象。若 KaTeX 是在首次渲染之后才加载完成，请重置一次 loader：
+
+```ts
+import { createKaTeXWorkerFromCDN, enableKatex, setKatexLoader, setKaTeXWorker } from 'markstream-vue'
+
+// 主线程使用 CDN 全局（UMD）
+setKatexLoader(() => (window as any).katex)
+enableKatex(() => (window as any).katex)
+
+// 可选：把 renderToString 放到 worker（worker 内部同样从 CDN 加载 KaTeX）
+const { worker } = createKaTeXWorkerFromCDN({
+  mode: 'classic',
+  katexUrl: 'https://cdn.jsdelivr.net/npm/katex@0.16.22/dist/katex.min.js',
+  mhchemUrl: 'https://cdn.jsdelivr.net/npm/katex@0.16.22/dist/contrib/mhchem.min.js',
+})
+if (worker)
+  setKaTeXWorker(worker)
+```
+
+若没有安装 KaTeX，数学表达式将被保留为纯文本以保证 SSR 安全。
+
+注意：在撰写源码 Markdown 时，请务必在 TeX 括号定界符前使用字面（转义）反斜杠。
+也就是说应写成 `\\(...\\)` 而不是 `\(...\)`，以便解析器能够可靠地识别行内 TeX。
+未转义的 `\(...\)` 无法与普通括号区分，可能不会被解析为数学公式。