Replace input field with project selector

Scope

Improve flow selection UI with enhanced dropdown components

Summary

Following the implementation of basic flow selection UI in #557560 (closed), this issue focuses on replacing the simple input fields with more user-friendly dropdown selectors that provide better user experience and data validation.

Scope

• Replace the basic project ID and group ID input fields with dropdown selectors
• Implement backend API integration to fetch and populate dropdown options with:
  • Available projects the user has access to
  • Available groups the user has access to
• Ensure dropdown selectors only display objects where the user has appropriate permissions
• Add proper loading states while fetching dropdown data from the backend
• Include search/filter functionality within dropdowns for better usability with large datasets
• Maintain the type selector functionality ('project' or 'group') but potentially integrate it with the dropdown selection flow

More Context

There is a discussion that there will be an allowlist. Task of this issue is to check whether this allowlist is relevant for this issue.