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/playground-react18/package.json

@@ -0,0 +1,29 @@
+{
+  "name": "playground-react18",
+  "type": "module",
+  "version": "0.0.0",
+  "private": true,
+  "scripts": {
+    "dev": "vite",
+    "build": "tsc -b --noEmit && vite build",
+    "preview": "vite preview"
+  },
+  "dependencies": {
+    "@iconify/react": "^4.1.1",
+    "markstream-react": "workspace:*",
+    "react": "^18.3.1",
+    "react-dom": "^18.3.1"
+  },
+  "devDependencies": {
+    "@types/react": "^18.3.28",
+    "@types/react-dom": "^18.3.7",
+    "@vitejs/plugin-react-swc": "^3.11.0",
+    "autoprefixer": "^10.4.27",
+    "monaco-editor": "^0.52.2",
+    "postcss": "^8.5.8",
+    "tailwindcss": "^3.4.19",
+    "typescript": "^5.9.3",
+    "vite": "^7.3.1",
+    "vite-plugin-monaco-editor-esm": "^2.0.2"
+  }
+}