Add pay ai agent page route

Add a new PayAIAgent page and route, wire navigation from EmployeeHeader to a live in-app page, and adjust header link behavior to navigate within the app (instead of opening a new tab). Update App routes to include /pay-ai-agent and implement a blank page placeholder for now. X-Lovable-Edit-ID: edt-6a67e0f1-8311-4f92-b4f8-cd47667838a0