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/src/styles/main.css

@@ -0,0 +1,32 @@
+@import '../../../themes/index.css';
+
+@tailwind components;
+@tailwind utilities;
+
+
+@layer components {
+  @import 'markstream-vue/index.css';
+}
+
+html,
+body,
+#app {
+  height: 100%;
+  margin: 0;
+  padding: 0;
+}
+
+/* Provide defaults for Tailwind gradient position variables so var() calls resolve */
+:root {
+  --tw-gradient-from-position: 0%;
+  --tw-gradient-to-position: 100%;
+}
+
+html.dark {
+  background: #121212;
+  color: #e0e0e0;
+}
+html{
+  color: #334155;
+}
+