Let menu entries appear as buttons with icon, colours and phone placement

The menu editor gets "Als Button darstellen" per entry: shape, appearance and
size from the button design, fill and ink with hover and palette, icon colour,
icon from the dialog, motion, icon-only and where the button sits on the phone
(in the opened menu, beside the burger, or there as an icon only). Colours go
into the page stylesheet under a stable per-button id because the CSP forbids
inline styles. ADR 110.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

templates/nav.html.twig

@@ -1,8 +1,10 @@
 {# One level of a resolved menu. Dropdowns use details/summary so submenus work without JavaScript. #}
 <ul class="menu-list menu-level-{{ level|default(1) }}">
 {% for link in links %}
-    <li>
-    {% if link.children is not empty %}
+    <li{% if link.button is defined %} class="menu-item-button{{ link.button.mobile|default('menu') in ['bar', 'bar-icon'] ? ' menu-keep-visible' }}"{% endif %}>
+    {% if link.button is defined %}
+        {% include 'menu-button.html.twig' with {link: link} only %}
+    {% elseif link.children is not empty %}
         <details class="menu-branch">
             <summary>{% include 'nav-label.html.twig' with {link: link} only %}</summary>
             <div class="menu-branch-body">