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/pnpm-workspace.yaml

@@ -0,0 +1,11 @@
+packages:
+  - playground
+  - playground-angular
+  - playground-next14
+  - playground-next15
+  - playground-vue2
+  - playground-vue2-cli
+  - playground-nuxt
+  - playground-react18
+  - playground-react19
+  - packages/*