The browser integration /auth endpoint currently returns plain text responses like "Authentication successful, you can close this window now." and "Authentication cancelled, you can close this window now."

These could be replaced with simple styled HTML pages that provide a better user experience:
  

• Picard/MusicBrainz branding (logo, consistent colors)
• Attempt window.close() via JavaScript to auto-close the tab after a delay, with a fallback message if the browser blocks it
• Clear visual distinction between success and failure/cancellation states (e.g. green checkmark vs. warning icon)