Behaviors & Constraints

A few product rules and UI behaviors worth knowing:

• Built-in templates (professional-modern, minimalist-clean) cannot be deleted.
• Customer deletion is blocked when invoices exist; UI redirects to guidance page.
• Overdue badge shows when the due date is in the past and status is not paid.
• Frontend proxies HTML/PDF to preserve auth cookies and add safer headers.
• Breadcrumbs shown on sub-pages; mobile-friendly navigation.