[sneridagh]

Part of #269. Stacked on #351.

The control panels are translated the way @plone/plate is, so they work the same in Plone Aurora and in Volto, and they ship translations: English, German, Spanish, Italian, Catalan and Brazilian Portuguese. A new conceptual guide documents the pattern both packages follow.

@plone/controlpanels

• Plate's i18n contract: ControlPanelsProvider takes a t function in react-i18next's shape, t(key, { defaultValue, ...values }), instead of translate. Aurora passes react-i18next's t as is. Apps on react-intl, such as Volto, pass translationFromIntl(intl), which turns the placeholders into ICU's. The screens keep using useTranslate() with their messages, so they don't change.
• Placeholders use i18next's double braces, {{count}}, in all 258 messages.
• No double escaping: the screens ask i18next not to escape the values (interpolation: { escapeValue: false }), since React escapes what it renders. Without it, "Intranet/Extranet" showed as Intranet/Extranet in the workflow dialog; the acceptance tests caught it.
• Catalogs in src/locales/<language>/common.json, i18next's common namespace, next to the package's main, where the registry looks:
  • en is generated from the messages in the code: pnpm i18n.
  • de, es, it, ca and pt-BR translate every message, with the terms of Volto's catalog for each language (Content type, Workflow, Fieldset, Short name…).
  • src/locales.test.ts fails when en is outdated, or when a translation has unknown keys or other placeholders.
• An add-on entry point: a default-exported applyConfig that returns the config, so Aurora can register the package and load its translations.
• A badge message: "Not globally addable" on the type page gets its own message. It shared one with the list's sentence about new types, which needs a plural in Spanish, Italian and Catalan.

Aurora

• apps/aurora depends on @plone/controlpanels and lists it in registry.config.ts, so the registry merges its catalogs into public/locales.
• cmsui's ContentTypesProviders passes react-i18next's t, and no longer fills in the placeholders itself.

Documentation

• New conceptual guide, docs/conceptual-guides/packages-for-volto-and-aurora.md, on what makes a package run in both apps:
  • no framework imports
  • a provider for app services, and a driver with capabilities for data
  • translations
  • scoped styles for Volto
  • shipping TypeScript source
  • @plone/registry as a peer dependency
• AGENTS.md: the root file summarizes the pattern and points to the guide; @plone/plate's and @plone/controlpanels' files point to it too.

Not in this PR

• Volto: Add the content types control panel from @plone/controlpanels volto#8500 will pass translationFromIntl(intl), and bring the messages into Volto's catalogs from these JSON files, as volto-plate does for Plate.
• Aurora's language choice doesn't apply: with more than English in supportedLanguages, choosing German in the personal preferences still renders English everywhere. app/i18n.ts reads the supported languages when it's first imported, probably before the configuration has run. That's a separate fix. Here I checked that Aurora serves the control panel's catalogs, for example /locales/de/common.json with "Inhaltstypen".

Checks

• @plone/controlpanels: 128 unit tests, including the contract, translationFromIntl and the catalogs. check:ts, lint and Prettier pass.
• @plone/cmsui: 451 unit tests; check:ts passes.
• Acceptance, content-types.test.ts against plone.restapi issue-2061-workflows: 23 passed, 2 skipped (the known gaps on that backend).
• Docs: make docs-html builds without warnings from the new guide.

See #269 for how to review and merge the stack.