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/workflows/docs-check.yml

@@ -0,0 +1,22 @@
+name: Docs check
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  build-docs:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install pnpm
+        uses: pnpm/action-setup@v2
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: pnpm
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+      - name: Build docs
+        run: pnpm docs:build:ci