Add Mark as Paid button for PARTIAL/UNPAID invoices

[EarlVadim]

This fork adds a Mark as Paid button (✓) for invoices with PARTIAL or UNPAID status in the WebUI
transactions table. Clicking the button changes the invoice status to PAID and triggers the payment callback
notification, allowing manual confirmation of payments received outside the system.

The button appears in the Status column of the Transactions table. Clicking it:

1. Changes the invoice status to PAID
2. Triggers the payment callback notification

This is useful when a partial payment is received and you want to manually confirm the invoice as fully paid.