Add an own-HTML block, and build the DEEG offers page on the embed block

Umut asked for a block that takes custom HTML, and for an iframe on a new
DEEG offers page. Only one of those was missing.

New core block "Eigenes HTML" (document schema v24): one property, the
markup, up to 20.000 characters, written to the page unchanged. Changing it
is reserved for administrators and compared against what is stored - on
saving, on saving an area, and on restoring an older revision - exactly like
an own script. <script> and <iframe> are refused with the way to the right
place instead of being swallowed by the content security policy: scripts
belong in the script field, frames in the "Externer Inhalt" block, because
only that one asks for consent and puts the host into frame-src (ADR 106).

DEEG: /angebote existed as a draft with placeholder text. It is now built
like the legal pages - small caps line, large light heading, lead on the
tinted band - with the IS24 result list below it and a line pointing to the
portal and to the contact form. Navigation and the offers band on the home
page point there. The consent question is switched on, so the frame loads
only after the visitor agrees, and both the banner and the placeholder carry
the site's own shapes and colours.

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

templates/blocks/block.html.twig

@@ -255,6 +255,14 @@
     </div>
     {% endif %}
     {% endif %}
+{% elseif type == 'html' %}
+    {# Markup des Betreibers, unveraendert. Geprueft wurde es beim Speichern (CustomCode::checkHtml),
+       setzen darf es nur ein Administrator - deshalb steht hier `raw` und keine Filterung. #}
+    {% if props.html|default('')|trim == '' %}
+    {% if editing|default(false) %}<p id="{{ block.id }}" class="block-unavailable{{ designClasses }}" role="note">Eigenes HTML: unter Inhalt den Code einfügen.</p>{% endif %}
+    {% else %}
+    <div id="{{ block.id }}" class="block-html{{ designClasses }}">{{ props.html|raw }}</div>
+    {% endif %}
 {% elseif type == 'postlist' %}
     <div id="{{ block.id }}" class="block-postlist{{ designClasses }}">
         {% set feed = postLists[block.id]|default([]) %}