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/generate-px-css.mjs

@@ -0,0 +1,45 @@
+import fs from 'node:fs/promises'
+import path from 'node:path'
+import process from 'node:process'
+
+const sourceCss = path.resolve('dist/index.css')
+const targetCss = path.resolve('dist/index.px.css')
+const pxPerRem = 16
+const remPattern = /(-?(?:\d+(?:\.\d+)?|\.\d+))rem\b/g
+
+function formatPx(value) {
+  if (value === 0)
+    return '0'
+
+  const rounded = Math.round(value * 10000) / 10000
+  return `${Number.parseFloat(rounded.toFixed(4))}px`
+}
+
+async function main() {
+  let css = ''
+  try {
+    css = await fs.readFile(sourceCss, 'utf8')
+  }
+  catch (err) {
+    const e = err
+    throw new Error(`[generate-px-css] Missing ${sourceCss}. Did the npm build run? (${e?.message || e})`)
+  }
+
+  let replacementCount = 0
+  const convertedCss = css.replace(remPattern, (_, remValue) => {
+    const rem = Number(remValue)
+    if (Number.isNaN(rem))
+      return `${remValue}rem`
+
+    replacementCount += 1
+    return formatPx(rem * pxPerRem)
+  })
+
+  await fs.writeFile(targetCss, convertedCss, 'utf8')
+  console.log(`[generate-px-css] Wrote ${targetCss} (${replacementCount} rem values converted using 1rem=${pxPerRem}px)`)
+}
+
+main().catch((err) => {
+  console.error(err)
+  process.exitCode = 1
+})