feature(unlock-app): Migrate checkout to app router

[0xTxbi]

Description

This PR introduces the migration of the checkout page from the pages directory to the app router for improved routing and performance consistency across the application.

Additionally, the heavy lifting related to optimizing paywall config parsing has already been implemented in PR #14698.

Issues

Fixes #
Refs #13673

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