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/public/_headers

@@ -0,0 +1,9 @@
+# Cloudflare Pages 重定向规则
+# 替代 Netlify 的 [[redirects]] 配置
+# https://developers.cloudflare.com/pages/configuration/headers/
+
+/*
+  X-Content-Type-Options: nosniff
+  X-Frame-Options: DENY
+  X-XSS-Protection: 1; mode=block
+  Referrer-Policy: strict-origin-when-cross-origin