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/docs/guide/docs-style.md

@@ -0,0 +1,20 @@
+# Docs assets & screenshots
+
+If you're adding screenshots or GIFs to the documentation, please follow these guidelines:
+
+- Keep file size small (use SVG where possible or optimized PNG/JPG/GIF)
+- Store assets in `docs/.vitepress/public/` so they are served as static files
+- Use descriptive filenames and include a caption under the image
+- Prefer SVG for diagrams and small screenshots
+
+Example usage in a markdown page:
+
+![Playground demo](/screenshots/playground-demo.svg)
+
+Thanks for making docs better! Please link the original playground page if you add a screenshot for reproducibility.
+
+Quick try — add an SVG screenshot (example):
+
+```md
+![Playground demo](/screenshots/playground-demo.svg)
+```