Update the footer to add the license.

lib/decentralised_book_index_web/components/my_partials/footer.ex

@@ -9,6 +9,9 @@ defmodule DecentralisedBookIndexWeb.Components.MyPartials.Footer do
         <span class="block text-sm text-gray-500 text-center dark:text-gray-400">
           © 2025 Kostiantyn Klochko. All Rights Reserved.
         </span>
+        <span class="block text-sm text-gray-500 text-center dark:text-gray-400">
+          <a href="https://git.kklochko.space/KKlochko/decentralised_book_index">Decentralised book index</a> is free software: you can redistribute it and/or modify it under the terms of <a href="https://www.gnu.org/licenses/agpl-3.0.en.html">the GNU Affero General Public License</a> as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.
+        </span>
       </div>
     </footer>
     """