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>

public/assets/site.css

@@ -570,6 +570,12 @@ html.kant-top-armed.kant-top-shown .scroll-top{opacity:1;visibility:visible;tran
 .block-legal{display:flex;flex-wrap:wrap;gap:6px;align-items:center;font-size:13px}
 .block-legal a{color:inherit;text-decoration:none}
 .block-legal a:hover{text-decoration:underline}
+/* Die Sprachwahl (ADR 111): eine kurze Reihe, die gewählte Sprache fett. Jedes Ziel mindestens
+   24 px, damit es mit dem Finger zu treffen ist. */
+.block-language ul{display:flex;flex-wrap:wrap;gap:2px;align-items:center;list-style:none;margin:0;padding:0}
+.block-language a{display:inline-flex;align-items:center;justify-content:center;min-width:24px;min-height:24px;padding:4px 8px;color:inherit;text-decoration:none;font-size:14px;line-height:1.2}
+.block-language a:hover{text-decoration:underline}
+.block-language a[aria-current]{font-weight:700}
 .image-zoom{cursor:zoom-in}
 .zoom-dialog{border:0;padding:0;max-width:min(96vw,1400px);max-height:94vh;background:transparent;color:#fff;overflow:visible}
 .zoom-dialog::backdrop{background:rgba(8,12,20,.88)}