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/copy-tailwind-css.mjs

@@ -0,0 +1,28 @@
+import fs from 'node:fs/promises'
+import path from 'node:path'
+import process from 'node:process'
+
+const fromDir = path.resolve('dist-tw')
+const toDir = path.resolve('dist')
+
+const fromCss = path.join(fromDir, 'index.tailwind.css')
+const toCss = path.join(toDir, 'index.tailwind.css')
+
+async function main() {
+  await fs.mkdir(toDir, { recursive: true })
+
+  try {
+    await fs.copyFile(fromCss, toCss)
+  }
+  catch (err) {
+    const e = err
+    throw new Error(`[copy-tailwind-css] Missing ${fromCss}. Did the tailwind build run? (${e?.message || e})`)
+  }
+
+  await fs.rm(fromDir, { recursive: true, force: true })
+}
+
+main().catch((err) => {
+  console.error(err)
+  process.exitCode = 1
+})