refactor(custom-metrics): read a refused save in one helper

Matching the name clash by its code grew the form component past its
size baseline. The refusal's sentence now comes from a helper beside it.

src/components/custom-metrics/custom-metric-form.tsx

@@ -60,6 +60,17 @@ interface CustomMetricFormProps {
   footerSlot?: HTMLElement | null;
 }
 
+/**
+ * The sentence for a refused save. A name clash is read by its code: a 409
+ * can also be the first save still running under the same key.
+ */
+function saveRefusal(err: unknown, t: (key: string) => string): string {
+  return err instanceof ApiError &&
+    err.meta?.errorCode === "customMetric.name_taken"
+    ? t("customMetrics.form.nameTaken")
+    : localizedApiError(err, t, "customMetrics.form.saveError");
+}
+
 /**
  * v1.25.5 — define or edit a custom metric.
  *
@@ -132,13 +143,7 @@ export function CustomMetricForm({
       );
       onSuccess?.(saved);
     } catch (err) {
-      // By its code: a 409 can also be the first save still running.
-      setError(
-        err instanceof ApiError &&
-          err.meta?.errorCode === "customMetric.name_taken"
-          ? t("customMetrics.form.nameTaken")
-          : localizedApiError(err, t, "customMetrics.form.saveError"),
-      );
+      setError(saveRefusal(err, t));
     } finally {
       setSubmitting(false);
     }