feat(admin): category editor auto-slug, SEO fill, breadcrumb and icon explainers

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

P0 user feedback: category icon input and breadcrumbs ('Хлебные крошки') were unexplained, slug and SEO were manual.

- Title edits derive the slug and SEO title with the same never-overwrite-manual-input rule as products (verified in browser)
- SEO group gains the same 'Fill from product details' one-click action
- Icon field now explains itself (emoji/symbol shown in menus) and shows a live preview of the entered symbol
- Breadcrumb block explains in merchant language what customers see (Home › Electronics › Phones) and where it comes from; separator changed to › to match the storefront
- New adminCategories keys (navBreadcrumbHint, iconHint) in en/ru/hy

src/app/i18n/translations.ts

@@ -1424,6 +1424,8 @@ export interface Translations {
     groupAdvanced: string;
     visibilityExplain: string;
     navBreadcrumb: string;
+    navBreadcrumbHint: string;
+    iconHint: string;
     navRoot: string;
     navChildren: string;
     navNoChildren: string;