Show Promotions on the Subscribe Modal

🔍 Problem Statement
As a publisher, I experience having no way to display a promotional message on the subscribe modal, which limits my ability to use urgency-driven offers to increase conversion during campaigns.

💡 User Story
As a publisher, I want to configure a promotional message or banner that appears on the subscribe modal, so that I can highlight time-limited offers and drive sign-ups.

🎯 Definition of Done
- Given a configured promotion, when the subscribe modal is displayed, then the promotion message is shown prominently.
- This change impacts the JS-SDK / ReactJS Elements (subscribe modal) and admin UI (promotion config).
- Limitations: v1 supports text-only promotions; countdown timers and images are out of scope.

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I’d like to be able to show a promotion on the subscribe modal.