Fixed admin-toolbar pnpm dev crash on empty umd/ (#28962)

no ref

`vite preview` exits immediately when its `outDir` doesn't exist, which
kills `concurrently --kill-others` and takes the whole `pnpm dev` Nx
fan-out down with it. Before
[#28936](#28936) a tracked
`umd/admin-toolbar.min.js` masked this on fresh clones; without it, any
fresh clone or `pnpm build:clean` reproduces it.

Fix: ensure `umd/` exists before `concurrently` launches preview.
Preview boots against an empty dir, serves 404 for the bundle for the
~1s before `build:watch` emits its first artifact, then starts serving
the real file. No upfront full build needed.

Verified locally: with `umd/` removed, `pnpm dev` produces a 200 on
`http://localhost:4176/admin-toolbar.min.js` by t=2s (was: immediate
crash + chain termination).

Peer public apps (portal, comments-ui, announcement-bar, sodo-search,
signup-form) avoid the same trap by running `pnpm build &&` upfront.
Left alone here — worth revisiting as a separate "template the
public-app dev scripts" pass.

Author: 9larsons

apps/admin-toolbar/package.json

@@ -20,7 +20,7 @@
   "scripts": {
     "build": "pnpm exec vite build",
     "build:watch": "pnpm exec vite build --watch --mode development",
-    "dev": "concurrently --kill-others --names preview,build \"pnpm exec vite preview -l silent\" \"pnpm build:watch\"",
+    "dev": "mkdir -p umd && concurrently --kill-others --names preview,build \"pnpm exec vite preview -l silent\" \"pnpm build:watch\"",
     "lint": "pnpm exec eslint src test --cache",
     "test": "pnpm test:unit",
     "test:unit": "pnpm run build && pnpm exec vitest run",