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/docs-style.md

@@ -0,0 +1,7 @@
+# 文档资源与截图（中文）
+
+如果向文档中添加截图或 GIF，请遵循以下建议：
+
+- 保持文件体积小（优先使用 SVG 或压缩图片）
+- 将资源放在 `docs/.vitepress/public/`，以便 VitePress 正确托管
+- 使用描述性文件名，并在图片下方添加说明