Default UI: Add Newsletter Selection on Registration

Problem Statement

As a digital publisher, I experience lack of newsletter opt-in behavior during registration, which results in lost opportunities for list growth and reduced audience engagement.

User Story

As a site visitor, I want to select newsletter preferences when I register, so that I can receive the types of content I care about.

As a publisher, I want newsletter options to appear automatically based on predefined settings, so that the experience is consistent and scalable.

Definition of Done (DoD)

A feature is done when:

1. If Pelcro.uiSettings.newsletters is defined, then during registration flows, dynamically render one checkbox per newsletter entry, using the label for display text and id for backend opt-in mapping.

2. Example config for newsletters here (in uiSettings)

3. Upon successful registration, the selected newsletters must be passed to Pelcro and synced with connected ESPs (e.g., Mailchimp) using Pelcro's backend logic.

4. If no newsletters are defined in uiSettings.newsletters, the registration form should not display any newsletter section.