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>

The playbook-shipped welcome.html hardcoded #2e2f32 for the headline and
welcome text, which is nearly invisible on the dark background Element
renders when the dark theme is active. Element Web's default
use_system_theme behavior keys the theme off the system color scheme, so
switch the text to Element's dark-theme text color (#ebeef2) under a
prefers-color-scheme: dark media query.

Fixes #4838

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

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/4445

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.

Fixes #586 (Github Issue)

Use Jinja2 lstrip_blocks option in templates to ensure consistent
indentation in generated files.