feat: improve admin build (#14485)

#### PR Dependency Tree


* **PR #14485** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Chores**
  * Admin static assets now served under /admin for self-hosted installs
  * CLI is directly executable from the command line
  * Build tooling supports a configurable self-hosted public path
  * Updated admin package script for adding UI components
* Added a PostCSS dependency and plugin to the build toolchain for admin
builds

* **Style**
* Switched queue module to a local queuedash stylesheet, added queuedash
Tailwind layer, and scoped queuedash styles for the admin UI

* **Bug Fixes**
  * Improved error propagation in the Electron renderer
* Migration compatibility to repair a legacy checksum during native
storage upgrades

* **Tests**
  * Added tests covering the migration repair flow
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/core/selfhost/static.ts

@@ -52,7 +52,7 @@ export class StaticFilesResolver implements OnModuleInit {
 
     // serve all static files
     app.use(
-      basePath,
+      basePath + '/admin',
       serveStatic(join(staticPath, 'admin'), {
         redirect: false,
         index: false,