Add Cancellation Reason in new UI

🔍 Problem Statement

Currently, when end users cancel their subscriptions, we do not collect structured data on the reasons for cancellation. This limits our ability to understand customer churn, identify patterns, and implement improvements to retain subscribers.

💡 User Story

As a business owner,

I want a required exit survey that asks customers their reason for cancelling via a dropdown with predefined options, plus an open-text field if “Other” is selected,

so that I can better understand why customers churn and use these insights to improve retention strategies.

🎯 Definition of Done (DoD)

A feature is considered done when:

• A required field is presented to users when they attempt to cancel a subscription.

• The field is a dropdown containing a list of reasons configured through the platform.

• If “Other” is selected, an open-text input field appears, allowing the user to specify their reason.

• The open-text input field is required when other is selected.

• The validations happens only on the UI strictly and not on the Open API.

• This change needs to be backward compatible with custom UIs and older react elements versions.

• The selected reason (and optional free-text entry) is stored in the subscription object and is accessible in subscription exports.