fix(i18n): localize API error messages by code (no more raw English)

Error toasts surfaced the raw English backend message. Added an errors namespace (fa/ar/en) keyed by error code + a useApiError() resolver that maps ApiClientError.code to the localized message (fallback to a localized generic). Wired into menu, tables, demo banner, and subscription checkout; hardened getErrorMessage so it never returns the raw backend message.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

web/dashboard/src/lib/notify.ts

@@ -46,7 +46,10 @@ export const notify = {
 };
 
 export function getErrorMessage(err: unknown, fallback: string): string {
-  if (err instanceof ApiClientError) return err.message;
+  // ApiClientError.message is the raw (usually English) backend message; prefer
+  // the caller's localized fallback. For code-specific localized text, use the
+  // useApiError() hook instead of this helper.
+  if (err instanceof ApiClientError) return fallback;
   if (err instanceof Error && err.message) return err.message;
   return fallback;
 }

web/dashboard/src/lib/use-api-error.ts

@@ -0,0 +1,21 @@
+import { useTranslations } from "next-intl";
+import { ApiClientError } from "@/lib/api/client";
+
+/**
+ * Returns a resolver that turns any caught error into a localized, user-facing
+ * message using the "errors" namespace. Known ApiClientError codes map to their
+ * translated message; otherwise the provided fallback is used, then a generic
+ * localized message. Never surfaces the raw (English) backend message.
+ *
+ *   const apiError = useApiError();
+ *   onError: (err) => notify.error(apiError(err))
+ */
+export function useApiError() {
+  const t = useTranslations("errors");
+  return (err: unknown, fallback?: string): string => {
+    if (err instanceof ApiClientError && err.code && t.has(err.code)) {
+      return t(err.code);
+    }
+    return fallback ?? t("generic");
+  };
+}