Allow separate mobile and desktop designs for popup CTAs

Problem: The popup CTA editor currently uses a single shared design for all devices (desktop, tablet, and mobile). There is no way to create a separate, mobile-optimized layout within the same CTA, nor is there a native option to show different popup CTA designs based on device type without creating entirely separate CTAs.

Impact: On mobile devices, popup CTAs often appear oversized and intrusive, negatively impacting user experience and conversion performance. Customers cannot optimize the mobile experience without also affecting the desktop version.

Requested Feature: Add the ability to design device-specific layouts (desktop vs. mobile) within the popup CTA editor, or provide native device-based design controls so that a single CTA can render differently depending on the visitor’s device.

Business Context: Competing platforms (e.g., Klaviyo) already offer this level of flexibility. This is a core capability expected at the Professional/Enterprise tier.