feat: allow overriding default English labels in admin bar

[Zayooo00]

The admin bar currently renders five hardcoded English strings inside the bar (Edit <singular>, New <singular>, Exit preview mode, Logout, Profile). There is no API for swapping them out short of forking the package, which makes the bar awkward for any site that runs in another language or wants different copy.

This PR adds an optional labels prop to PayloadAdminBar that lets consumers override each of those strings individually:

<PayloadAdminBar
  cmsURL="https://cms.website.com"
  collectionSlug="pages"
  collectionLabels={{ singular: 'strona', plural: 'strony' }}
  id="12345"
  labels={{
    edit: ({ singular }) => `Edytuj ${singular ?? 'stronę'}`,
    create: ({ singular }) => `Nowa ${singular ?? 'strona'}`,
    exitPreview: 'Wyjdź z podglądu',
    logout: 'Wyloguj',
    profile: 'Profil',
  }}
/>

Shape

labels?: {
  edit?: ((args: { singular?: string }) => string) | string
  create?: ((args: { singular?: string }) => string) | string
  exitPreview?: string
  logout?: string
  profile?: string
}

• edit and create accept either a plain string or a function. The function form receives the resolved collectionLabels.singular so the consumer can localize while still pulling the collection name into the label.
• Every entry is optional. Omitted entries fall back to the existing English defaults.

Backwards compatibility

No behavior change for existing consumers. All five fallbacks match the strings that were hardcoded before.

Notes

• README updated with a Customizing labels section and a Polish usage example.
• No new dependencies.
• Tested locally against this repo's pnpm exec tsc --noEmit.

If you'd prefer a different API shape (e.g. a flat editLabel / createLabel rather than a grouped labels object), happy to revise.