feat(10-02): backend strings, controller messages and declarative toolbar

- phrasebook ships the backend::lang admin strings (pl, en) with CLDR
  plural maps, loads them as namespace backend, applies
  pact.HasLangOverrides trees (lang/<locale>/<namespace>/<group>.yaml)
  after every namespace, and fails activation when a backend key cannot
  convert to plural forms
- Translator.Forms, Bundle, Resolved and Has serve keys as CLDR form maps
- Public GET /lang returns every backend::lang key for the request
  locale over the fallback locale, Cache-Control no-cache
- config_list, config_form and config_relation accept a strict messages
  block; omitted keys take framework defaults, schemas serve every message
  as CLDR forms, and activation fails on a missing phrase key
- toolbar.buttons is an ordered [create, delete] list; the Winter string
  form, duplicates, unknown actions and delete without showCheckboxes fail
  at boot, and create is dropped when the controller has no form
- Form schema serves the raw Winter redirects; scaffold emits the list
  syntax; form and relation schema routes are typed in the admin OpenAPI

pact/capabilities.go

@@ -268,6 +268,15 @@ type HasLang interface {
 	LangFS() fs.FS
 }
 
+// HasLangOverrides is implemented by plugins (usually the app plugin) that
+// replace or add translations of any loaded namespace, including the
+// framework's backend::lang admin strings, and may add locales (D-20).
+// Layout: lang/<locale>/<namespace>/<group>.yaml, e.g.
+// lang/pl/backend/lang.yaml.
+type HasLangOverrides interface {
+	LangOverridesFS() fs.FS
+}
+
 // HasMailTemplates is implemented by plugins that ship Winter-shaped mail
 // templates and layout aliases. MailTemplatesFS contains views/mail assets;
 // MailTemplates lists dotted template names; MailLayouts maps a short layout