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/stream-monaco/examples/streaming-demo/README.md

@@ -0,0 +1,22 @@
+# Streaming Demo (Vue 3 + Vite)
+
+A minimal example showing streaming append and language switching using `stream-monaco`.
+
+- Single editor with streaming `appendCode`
+- Switch language mid-stream with `setLanguage`
+- Optional: Diff editor streaming via `appendOriginal` / `appendModified`
+
+## Quick start
+
+```bash
+pnpm i
+pnpm -C examples/streaming-demo dev
+```
+
+Then open http://localhost:5173
+
+Routes:
+
+- `/` Streaming append demo
+- `/diff` Diff streaming stress demo
+- `/diff-ux` Diff UX demo (unchanged-region collapse/expand + hover hunk revert/stage)