Proof of concept: Add author form (T407551)

This merge (closes T407551 introduces a new page, add-author.html, which serves as a proof of concept for the future author creation feature in Paulina.

The new page:

Inherits Paulina’s standard header and footer for consistency.

Contains a basic web form with input fields for author information, including name, birth date, nationality, occupation, and description.

Includes a submit button (no backend functionality yet).

This serves as an initial step toward implementing Wikidata author creation functionality. Screen_Recording_2025-10-18_at_8.50