fix: category slug-uniqueness check fails closed on API error

isSlugTaken previously caught network/API errors and returned false,
letting the save proceed as if the slug were free. Now the error
propagates and blocks save via a distinct slugCheckError state,
surfaced in the category form with its own hint.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/translations.ts

@@ -1541,6 +1541,7 @@ export interface Translations {
     title: string;
     slug: string;
     slugTaken: string;
+    slugCheckError: string;
     parent: string;
     noParent: string;
     icon: string;