Add 'Pay in Full' Billing Option

The customer requests a new feature to be added under the billing section. This feature, labeled "Pay in full," would allow clients to pay their entire contract amount upfront via various methods (cash, card, check, etc.). This option should be separate from existing "Cycle monthly billing" or "After job completion billing" options. The primary purpose is to automatically mark all pending services as paid once the full upfront payment is processed, addressing a current lack of a clear tracking method for such payments.