Enhance Admin Dashboard for user email display and application handling

[rsmoke]

This pull request improves the robustness of the admin dashboard by handling cases where expected associations or methods may be missing, and adds a test to ensure correct display when a payment is not linked to an application. The changes prevent errors in the dashboard and enhance the user experience for admins.

Admin dashboard robustness improvements:

• Updated the "Recent Applications" panel to safely handle cases where Application.active_conference_applications may not return objects with a display_name method, preventing potential errors.
• Enhanced the "Recent Payments" panel to display the user's email with a message when a payment is not linked to a current application, instead of failing or omitting the entry.
• Improved the "Accepted Applicants" and "Waiting for Responses" panels to check for method existence before calling collection methods, and to provide fallback logic if methods are missing, increasing dashboard reliability.
• Wrapped the dashboard columns in a begin/rescue block to catch and silently handle NoMethodError exceptions, ensuring the dashboard remains accessible even if data is incomplete.

Testing:

• Added a request spec (spec/requests/admin_dashboard_spec.rb) to verify that the admin dashboard correctly displays the user's email with a message when a payment exists without a corresponding application.