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

internal/build/build_test.go

@@ -978,6 +978,8 @@ func TestMakeAdminControllerLayout(t *testing.T) {
 		for _, text := range []string{
 			"modelClass: Albums",
 			"list: ~/plugins/golem15/demo/models/albums/columns.yaml",
+			"buttons: [create]",
+			"prompt: backend::lang.list.search_prompt",
 		} {
 			if !bytes.Contains(listSrc, []byte(text)) {
 				t.Fatalf("config_list.yaml missing %s:\n%s", text, listSrc)