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/public/favicon.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 32 32">
+  <style>
+    path { fill: #222; }
+    @media (prefers-color-scheme: dark) {
+      path { fill: #ffffff; }
+    }
+  </style>
+  <path d="M27.562 26L17.17 8.928l2.366-3.888L17.828 4L16 7.005L14.17 4l-1.708 1.04l2.366 3.888L4.438 26H2v2h28v-2zM16 10.85L25.22 26H17v-8h-2v8H6.78z" />
+</svg>