Enhancement: connect Form Block to Payment Block

### What

It should be possible to integrate a Payment Block as a Form Block element, so that the Form Block could be used in contexts like paid event registration.

### Why

This is a behavior that customers already expect, based on interactions like 40933223-hc and 40960046-hc.

### How

The idea is to gate the Form Block submission to a Payment Block. I'm not sure what makes more sense:

- Clicking on "Submit" triggers a payment screen, or
- The "Submit" button is only enabled after the customer successfully completes a payment in a modal after clicking on the Payment Button.

Start by locating the Form Block and Payment Block entry points and reviewing how each currently handles submission and payment completion. Decide which interaction is supported, then verify that a paid form registration cannot submit before payment and that the chosen flow works in the relevant block contexts.

Written by the indexing model from the issue text.