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/en.ts

@@ -1533,6 +1533,7 @@ export const en: Translations = {
     title: 'Title',
     slug: 'URL slug',
     slugTaken: 'This slug is already used by another category.',
+    slugCheckError: 'Could not verify this slug is unique. Try again before saving.',
     parent: 'Parent category',
     noParent: 'No parent (top level)',
     icon: 'Icon',