fix: correct frontend asset paths

[badzim]

This PR fixes a path-resolution issue in the AdminJS router that causes all frontend bundles (app.bundle.js, global.bundle.js, design-system.bundle.js) to return 404 when AdminJS is installed with pnpm, especially in monorepos or virtual store setups.

Because the current code relies on __dirname + relative segments to locate lib/frontend/assets, the computed paths point to the wrong location under pnpm. As a result, the AdminJS UI does not load — this also affects the AdminJS demo site, which is currently down for the same reason.

This PR replaces the brittle relative paths with robust module-resolution logic:

const ASSETS_ROOT = path.join(
  process.cwd(),
  'node_modules',
  'adminjs',
  'lib',
  'frontend',
  'assets',
)

const resolveDesignSystemBundle = () => {
  return path.join(
    process.cwd(),
    'node_modules',
    '@adminjs',
    'design-system',
    `bundle.${NODE_ENV}.js`,
  )
}

This approach works reliably across:

• pnpm (hoisted and non-hoisted)
• npm / yarn
• monorepos / workspaces
• Vite / NestJS / Express integrations

With this fix, all asset URLs under /admin/frontend/assets/* resolve correctly again, and the AdminJS UI loads normally.

[badzim]

@dziraf what do you think ?