fix(cabana): translate hook ValidationError phrase keys in the request locale

- One shared localizer for ValidationError and ForbiddenError, tested in
  writeCRUDError's order; replaces the ForbiddenError-only one.
- Applied on the CRUD and relation service transactions, the detached bulk
  branch, runAction, the record read (ShowRecord), and the reorder,
  singleton and relation-options routes, which localized neither type.
- Only strings containing "::" are looked up, so translated text passes
  through unchanged and logs no missing-key warning.
- Copy-on-write: the plugin's error value is never modified.
- README and backend docs describe the localized ValidationError.

modules/cabana/singleton.go

@@ -103,7 +103,8 @@ func (s *service) singletonRoute(w http.ResponseWriter, r *http.Request) {
 			if !errors.As(err, &missing) {
 				slog.Error("cabana: singleton record failed", "controller", controllerID(cc), "error", err)
 			}
-			writeCRUDError(w, err)
+			// A refusal from SingletonRecord carries plugin phrase keys.
+			writeCRUDError(w, localizeControllerError(ctx, s.translator(), err))
 			return
 		}
 		WriteData(w, http.StatusOK, result, nil)