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>

src/Content/PartialRepository.php

@@ -98,7 +98,7 @@ final class PartialRepository
         if ('section' !== $kind && null === $this->findByKind($kind, 'default', $locale)) {
             $slug = 'default';
         }
-        $document = $this->richText->normaliseDocument($document);
+        $document = Translations::normalise($this->richText->normaliseDocument($document), $this->richText);
         $this->validator->validate($document, false, true);
         if (null !== $this->findByKind($kind, $slug, $locale)) {
             throw new InvalidPage(sprintf('„%s“ ist als Adresse bereits vergeben.', $slug));
@@ -116,7 +116,7 @@ final class PartialRepository
 
     public function save(string $id, array $document, int $expectedVersion): void
     {
-        $document = $this->richText->normaliseDocument($document);
+        $document = Translations::normalise($this->richText->normaliseDocument($document), $this->richText);
         $this->validator->validate($document, false, true);
         $this->db->transactional(function () use ($id, $document, $expectedVersion): void {
             // A section must not end up containing itself, directly or through another section.