custom templates when editing pages in Admin

I created a new page template for use with a plugin, eg., `sequence`

When I tried to make a minor edit to the page in the Admin page, the file name was changed to `blog.en-gb.md`, and consequently, I got errors.

The drop-down menu in the Admin page does not contain `sequence` as a valid template. The template is correctly recognised and rendered in Grav.

Reproduce the issue in the Admin page with a page using the custom sequence template, then trace the template selection and save flow that changes the filename to blog.en-gb.md. Done means sequence is available as a valid template in the dropdown, editing preserves the intended filename, and Grav still recognises and renders the page.

Written by the indexing model from the issue text.