fix: interview invitation email not sending - add auth headers and error handling

- Frontend: Added Authorization Bearer token from Supabase session to the /interviews/schedule API call (was missing, causing auth failures)
- Frontend: Fixed error response parsing to match backend's structured { error: { message } } format
- Frontend: Added diagnostic console logging for email send attempts
- Backend: Wrapped sendEmail calls in try/catch blocks to prevent unhandled 500 crashes
- Backend: Returns structured error with EMAIL_ERROR code when Resend API fails
- Backend: Added detailed logging for email delivery diagnostics

Author: simpaticohr