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

cabana/lang.go

@@ -0,0 +1,24 @@
+package cabana
+
+import (
+	"net/http"
+)
+
+// adminLangPrefix is the only namespace the public bundle serves (D-20).
+const adminLangPrefix = "backend::lang."
+
+// langBundle serves GET {prefix}/api/v1/lang: every backend::lang key as CLDR
+// forms for the request locale, with the fallback locale's keys underneath.
+// It is public (the login screen needs it) and never includes another
+// namespace.
+func (s *service) langBundle(w http.ResponseWriter, r *http.Request) {
+	tr := s.translator()
+	locale := schemaLocale(r.Context(), tr)
+	data := map[string]map[string]string{}
+	if tr != nil {
+		locale = tr.Resolved(locale, adminLangPrefix)
+		data = tr.Bundle(locale, adminLangPrefix)
+	}
+	w.Header().Set("Cache-Control", "no-cache")
+	WriteData(w, http.StatusOK, data, map[string]any{"locale": locale})
+}