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/.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,19 @@
+## Summary
+
+Please explain the goal of this change in 2–3 lines.
+
+## Changes
+
+- [ ]
+
+## Screenshots / Demo (if UI/behavior changes)
+
+- [ ] Added GIF/screenshot
+- [ ] Shareable repro link from https://markstream-vue.simonhe.me/test (recommended)
+
+## Checklist
+
+- [ ] Lint: `pnpm lint`
+- [ ] Typecheck: `pnpm typecheck`
+- [ ] Tests: `pnpm test` (or `pnpm test:update` if snapshots changed)
+- [ ] Build: `pnpm build` (library + CSS)