fix(cabana): send the default content locale first in enabledLocales

- enabledContentLocales moves TranslationWriter.DefaultLocale to index 0
  and keeps the other enabled codes in the writer's order
- every ML control opens on enabledLocales[0], so the form now shows the
  host-column values (content, excerpt, title, slug) instead of an empty
  non-default locale
- a DefaultLocale error is logged and keeps the writer's order
- README and forms docs: default locale first

modules/cabana/http.go

@@ -980,6 +980,17 @@ func (s *service) enabledContentLocales(ctx context.Context) []string {
 		slog.Error("cabana: enabled locales", "error", err)
 		return nil
 	}
+	// The SPA opens every ML control on the first code, and the default
+	// locale's values live in the host columns, so the default goes first
+	// and the other codes keep the writer's order.
+	def, err := writer.DefaultLocale(ctx, db)
+	if err != nil {
+		slog.Error("cabana: default locale", "error", err)
+		return codes
+	}
+	if i := slices.Index(codes, def); def != "" && i > 0 {
+		codes = append([]string{def}, slices.Delete(slices.Clone(codes), i, i+1)...)
+	}
 	return codes
 }