[Frontend] Role-based permissions for Convert to GitLab CI/CD Flow

Summary

Integrate frontend permission checks for the "Convert to GitLab CI/CD" flow based on minimum_access_level_execute_asynchronously permission.

Background

Convert to GitLab CI/CD Flow is a foundational background flow that requires Developer+ role. Users should only see the "Convert to GitLab CI/CD" button when viewing a Jenkinsfile if they have the appropriate execute permission.

Requirements

• Hide/disable "Convert to GitLab CI/CD" button when user lacks permission
• Check permission when viewing Jenkinsfile in project
• Show appropriate message when permission is denied
• Ensure UI updates reflect permission state

Locations

• Jenkinsfile page > "Convert to GitLab CI/CD" button (above file content)

Acceptance Criteria

• Button hidden/disabled for users without Developer+ role or custom permission
• Permission check respects instance and namespace-level settings
• Clear messaging when access is denied

Related

• Parent Epic: &19742
• Backend: #578553