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-next15/app/page.tsx

@@ -0,0 +1,16 @@
+import Link from 'next/link'
+
+export default function HomePage() {
+  return (
+    <main className="home-shell">
+      <h1>Next 15 Playground</h1>
+      <p>Use the dedicated SSR lab routes for formal acceptance.</p>
+      <nav>
+        <ul>
+          <li><Link href="/app-ssr-lab">/app-ssr-lab</Link></li>
+          <li><Link href="/pages-ssr-lab">/pages-ssr-lab</Link></li>
+        </ul>
+      </nav>
+    </main>
+  )
+}