This makes the Registered Apps page more user-friendly for new users who haven't connected any Mastodon apps yet, by showing
helpful onboarding instructions instead of an empty table.

1. Empty state for "Registered Apps" page (templates/registered-apps.php)

• Previously, the page always showed the apps table even when empty
• Now it shows a helpful "No apps have been registered yet" message with instructions on how to get started when there are no apps
• The help box includes a link to the Mastodon apps directory and displays the instance URL that users need to enter in their Mastodon
  app

2. Pass instance URL to the template (includes/class-mastodon-admin.php:631)

• Added instance_url to the template args so it can be displayed in the new empty state help box

3. CSS styling for the help box (admin.css:185-186)

• Extended the existing input styling to also apply to inputs in the .help-box on the registered apps page, so the instance URL input
  field has the same styling as on the welcome page

4. Better link anchor (templates/welcome.php:24)

• Updated the Mastodon apps directory link to include a URL fragment (#:~:text=Browse%20third-party%20apps) that scrolls users directly
  to the "Browse third-party apps" section on the destination page