fix(unlock-app): Improve migration checkout experience

[0xTxbi]

Description

This PR introduces functionality for users to sign out before completing the migration process. Additionally, it ensures that users are authenticated in Locksmith upon completing a successful migration, building upon the changes in PR #15080.

Issues

Fixes #
Refs #15080

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