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/katex-worker-performance-analysis.md

@@ -0,0 +1,24 @@
+// Copied KaTeX zh page to the guide subfolder for proper zh routing
+# KaTeX Worker 性能分析指南（中文）
+
+## 问题：Worker 是否比直接渲染更合适？
+
+本文档帮助你判断在何种场景下使用 Web Worker 会带来性能优势。
+
+## 快速结论
+
+**是的，Worker + Cache 架构在大多数真实场景中有显著优势。**
+
+关键原因：
+1. Cache 能消除大部分重复渲染开销（缓存命中率通常 >70%）
+2. Worker 能阻止主线程被阻塞，从而保证 UI 响应
+3. 内存占用极小（举例：200 个公式大约 10–50KB）
+
+## 简要对比（场景）
+
+- 场景：单个简单公式
+  - 直接渲染：约 2–5ms
+  - Worker：约 3–7ms（含通信开销）
+  - 结论：Worker 略慢，但差异可以忽略
+
+... (rest of content copied from top-level zh KaTeX) ...