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/scripts/build-workers-rollup.mjs

@@ -0,0 +1,47 @@
+#!/usr/bin/env node
+import fs from 'node:fs'
+import path from 'node:path'
+import process from 'node:process'
+import typescript from '@rollup/plugin-typescript'
+import { rollup } from 'rollup'
+
+const root = path.resolve(new URL(import.meta.url).pathname, '..', '..')
+const outdir = path.resolve(root, 'dist', 'workers')
+
+async function buildWorker(entry, outName) {
+  const bundle = await rollup({
+    input: entry,
+    external: ['mermaid', 'katex'],
+    plugins: [
+      typescript({ tsconfig: path.resolve(root, 'tsconfig.json'), sourceMap: false }),
+    ],
+  })
+
+  await bundle.write({
+    dir: outdir,
+    format: 'es',
+    entryFileNames: outName,
+    chunkFileNames: '[name].js',
+    sourcemap: false,
+  })
+
+  await bundle.close()
+}
+
+async function run() {
+  try {
+    if (!fs.existsSync(outdir))
+      fs.mkdirSync(outdir, { recursive: true })
+
+    await buildWorker(path.resolve(root, 'src/workers/katexRenderer.worker.ts'), 'katexRenderer.worker.js')
+    await buildWorker(path.resolve(root, 'src/workers/mermaidParser.worker.ts'), 'mermaidParser.worker.js')
+
+    console.log('Workers written to', outdir)
+  }
+  catch (err) {
+    console.error('Failed to build workers with rollup', err)
+    process.exit(1)
+  }
+}
+
+run()