feature(unlock-app): Migrate account checkout

[0xTxbi]

Description

Enable account migration on checkout page

This PR introduces the ability for users to directly migrate their accounts from within the checkout page. This streamlines the user experience by reducing steps required for migration, allowing seamless account transitions at the point of purchase.

Issues

Fixes #
Refs #14858

Checklist:

• 1 PR, 1 purpose: my Pull Request applies to a single purpose
• I have commented my code, particularly in hard-to-understand areas
• I have updated the docs to reflect my changes if applicable
• I have added tests (and stories for frontend components) that prove my fix is effective or that my feature works
• I have performed a self-review of my own code
• If my code involves visual changes, I am adding applicable screenshots to this thread