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-vue2/tailwind.config.js

@@ -0,0 +1,17 @@
+/** @type {import('tailwindcss').Config} */
+export default {
+  darkMode: ['selector'],
+  safelist: [
+    'border-gray-400/5',
+    'hover:bg-[var(--vscode-editor-selectionBackground)]',
+  ],
+  content: [
+    './src/**/*.{vue,js,ts,jsx,tsx}',
+    '../packages/markstream-vue2/src/**/*.{vue,js,ts,jsx,tsx}',
+  ],
+  theme: {
+    extend: {
+    },
+  },
+  plugins: [],
+}