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-react19/tailwind.config.js

@@ -0,0 +1,13 @@
+import baseConfig from '../tailwind.config.js'
+
+const baseContent = Array.isArray(baseConfig.content) ? baseConfig.content : []
+
+export default {
+  ...baseConfig,
+  content: [
+    './index.html',
+    './src/**/*.{js,ts,jsx,tsx}',
+    '../packages/markstream-react/src/**/*.{js,ts,jsx,tsx,css}',
+    ...baseContent,
+  ],
+}