Element Web redesigned its welcome page into a built-in component
(element-hq/element-web#33211, first released in spring 2026) and no
longer falls back to loading welcome.html. The custom page the playbook
installed had therefore silently stopped having any effect, and it was a
fork of an upstream file that upstream has deleted (its button icon
assets are already missing from current Element Web builds).

Remove the welcome.html shipping and the variables that only applied to
it (matrix_client_element_welcome_headline, _welcome_text,
_welcome_logo_link, _page_template_welcome_path), rejecting them in
validate_config.yml with pointers to the alternatives. Logo and
background customization keep working through Element Web's branding
options, which the new welcome page still honors. A fully custom page
can be self-hosted and wired via embedded_pages.welcome_url.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>