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/contributing.md

@@ -0,0 +1,26 @@
+# 贡献指南
+
+欢迎贡献！
+
+常用开发命令：
+
+- `pnpm dev` / `pnpm play` — 启动 playground
+- `pnpm build` — 构建库
+- `pnpm build:analyze` — 生成 bundle 可视化报告
+- `pnpm size:check` — 执行包体积预算检查
+- `pnpm test` — 运行测试
+- `pnpm docs:dev` — 启动文档站点
+
+体积预算环境变量（可选）：
+
+- `MAX_DIST_BYTES`
+- `MAX_JS_CHUNK_BYTES`
+- `MAX_PACK_TGZ_BYTES`
+- `MAX_PACK_UNPACKED_BYTES`
+
+文档贡献：
+
+1. 新页面写入 `docs/` 或 `docs/zh/`
+2. 更新 `docs/.vitepress/config.ts` 的侧边栏
+3. 运行 `pnpm docs:dev` 预览
+4. 提交 PR 并等待审阅