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-angular/index.html

@@ -0,0 +1,17 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8">
+    <meta name="viewport" content="width=device-width, initial-scale=1.0">
+    <link rel="icon" href="/favicon.svg" type="image/svg+xml">
+    <meta
+      name="description"
+      content="markstream-angular playground for streaming Markdown, Mermaid, KaTeX, and cross-framework rendering checks."
+    >
+    <title>markstream-angular Playground | Angular demo</title>
+  </head>
+  <body>
+    <playground-angular-root></playground-angular-root>
+    <script type="module" src="/src/main.ts"></script>
+  </body>
+  </html>