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-cli/public/index.html

@@ -0,0 +1,22 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8" />
+    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
+    <meta name="viewport" content="width=device-width,initial-scale=1.0" />
+    <link rel="icon" href="<%= BASE_URL %>favicon.svg" type="image/svg+xml" />
+    <meta
+      name="description"
+      content="markstream-vue2 Vue CLI playground for Vue 2.6 and Webpack 4 compatibility testing."
+    />
+    <title>markstream-vue2 Playground | Vue CLI 4 / Webpack 4</title>
+  </head>
+  <body>
+    <noscript>
+      <strong>This app requires JavaScript to run.</strong>
+    </noscript>
+    <div id="app"></div>
+    <!-- Load Mermaid as a global for Webpack 4 compatibility (avoid `exports`-only ESM entry). -->
+    <script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
+  </body>
+</html>