Add offline transfer application settings (admin level)

Summary

Add admin-level application settings to control offline transfer exports and imports, similar to other importer settings in admin/application_settings/general.

As part of the offline transfer feature for semi-automated migrations between offline instances (&8985), administrators need to be able to enable/disable offline transfer functionality at the instance level.

Backend

• Create application settings to gate offline transfer exports
• Create application settings to gate offline transfer imports
• Settings should be stored in admin/application_settings/general alongside other importer settings
• Settings should be retrievable and updatable via API

Frontend

• Add UI controls in admin settings panel to manage offline transfer settings
• Display toggles/controls for (these can be refined further with Technical Writing)
  • "Allow exporting GitLab groups and projects by offline transfer"
  • "Allow importing GitLab groups and projects by offline transfer"
• Add tasks to the documentation placeholder issue #581225

Notes

• These settings should follow the same pattern as other importer settings already in the application
• Settings should control both the API availability and UI visibility of offline transfer features

Related Issues

• &8985 - Support semi-automated migrations between offline instances

Acceptance Criteria

• Application settings for offline transfer exports are created and stored
• Application settings for offline transfer imports are created and stored
• Settings can be retrieved and updated via API
• Admin UI displays offline transfer settings in admin/application_settings/general
• Settings properly gate the offline transfer functionality
• Documentation is planned for before the API release and the subsequent UI release