Translate pages, areas, menus, forms and posts field by field instead of rebuilding them

A site can have further languages (Settings → Languages). A page is built once; each language
adds only its texts, keyed by the stable block IDs, plus its own address under /en/… (ADR 111,
document schema v27, migration 20261009000100 for page_translation).

- Editor: DE | EN switch, a list of all texts with original, state (missing, translated,
  original changed) and filter; structure stays with the main language. Undo, drafts,
  publishing and revisions cover all languages at once.
- Website: /en and translated slugs, lang, canonical, hreflang with x-default, og:locale,
  sitemap; links to translated pages follow the language; a language switch block.
- Header, footer and linked sections translate the same way; menu entries carry labels per
  language; fixed template texts come from a small dictionary.
- Forms: labels, help, options, button and success message per language; messages and the
  step script speak the language, the submission and the email stay in the main language.
- Posts, archives, category names, consent text and the 404 page follow the language.
- Single-language sites see none of it and never touch the new table.

The forms administration becomes an overview with one row per form and its own edit page;
fields are one line each with details collapsed.

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

templates/admin/menus.html.twig

@@ -8,7 +8,7 @@
 {% block content %}
 {% include 'admin/menu-icons.html.twig' %}
 <nav class="menu-slots" aria-label="Menü auswählen">{% for key, label in slots %}<a href="{{ path('admin_menus', {slot: key}) }}"{{ key == slot ? ' aria-current="page"' : '' }}>{{ label }}</a>{% endfor %}</nav>
-<form action="{{ path('admin_menu_save', {slot: slot}) }}" method="post" id="menu-editor" class="menu-workspace menu-form" data-icons="{{ menuIcons|json_encode|e('html_attr') }}" data-button-schema="{{ buttonSchema|json_encode|e('html_attr') }}">
+<form action="{{ path('admin_menu_save', {slot: slot}) }}" method="post" id="menu-editor" class="menu-workspace menu-form" data-icons="{{ menuIcons|json_encode|e('html_attr') }}" data-button-schema="{{ buttonSchema|json_encode|e('html_attr') }}" data-languages="{{ languages|json_encode|e('html_attr') }}">
     <input type="hidden" name="_csrf_token" value="{{ csrf_token('menus') }}"><input type="hidden" name="expectedVersion" value="{{ menu ? menu.version : 0 }}">
     <aside class="menu-add" aria-label="Menüeinträge hinzufügen">
         <section class="card card-body"><h2>Seiten und Beiträge</h2>