fix: Bind the page layouts to the front-end theme

Problem/Motivation

The page layouts of the template are built with the components of UI Suite UIkit, and they also wrap pages that the administration theme renders, such as the log in page and the account form. Those pages come out unstyled.

Steps to reproduce

1. Install a site from the template.
2. Open /user/login, or /user/1/edit as a signed-in user.
3. The page shows the front-end page layout inside UIkit Admin.

Proposed resolution

• Add a current_theme condition to each conditional page layout.
• Document the step for installed sites in the README.

Remaining tasks

• ✔ File the issue
• ➖ Merge request with a green pipeline
• ➖ Prove on a fresh install and on an updated site

User interface changes

N/A

API changes

N/A

Data model changes

N/A

AI-Generated: Yes