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.mjs

@@ -0,0 +1,36 @@
+#!/usr/bin/env node
+import path from 'node:path'
+import process from 'node:process'
+import { fileURLToPath } from 'node:url'
+import { build } from 'esbuild'
+
+const __filename = fileURLToPath(import.meta.url)
+const __dirname = path.dirname(__filename)
+
+const outdir = path.resolve(__dirname, '../dist/workers')
+
+async function run() {
+  try {
+    await build({
+      entryPoints: [
+        path.resolve(__dirname, '../src/workers/katexRenderer.worker.ts'),
+        path.resolve(__dirname, '../src/workers/mermaidParser.worker.ts'),
+      ],
+      bundle: true,
+      format: 'esm',
+      platform: 'browser',
+      outdir,
+      sourcemap: false,
+      minify: true,
+      external: ['mermaid', 'katex'],
+      target: ['es2017'],
+    })
+    console.log('Workers built to', outdir)
+  }
+  catch (err) {
+    console.error('Failed to build workers', err)
+    process.exit(1)
+  }
+}
+
+run()