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/registry.go

@@ -77,6 +77,10 @@ func compileRegistry(items []controllerRef) (*Registry, error) {
 		if err != nil {
 			return nil, err
 		}
+		if form == nil && list != nil {
+			// Without a compiled form there is nothing to create (D-14).
+			list.ToolbarButtons = withoutAction(list.ToolbarButtons, "create")
+		}
 		compiled := &CompiledController{
 			PluginID:       item.plugin.ID(),
 			Controller:     item.ctl,
@@ -93,6 +97,16 @@ func compileRegistry(items []controllerRef) (*Registry, error) {
 	return &Registry{byID: byID}, nil
 }
 
+func withoutAction(actions []string, drop string) []string {
+	out := make([]string, 0, len(actions))
+	for _, action := range actions {
+		if action != drop {
+			out = append(out, action)
+		}
+	}
+	return out
+}
+
 func compileContributions(reg *Registry, plugins []party.Plugin) error {
 	if reg == nil {
 		return fmt.Errorf("cabana: registry is nil")