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/en.ts

@@ -1417,6 +1417,8 @@ export const en: Translations = {
     groupAdvanced: 'Advanced',
     visibilityExplain: 'A hidden category and its products stay out of the storefront navigation and search until you make it visible again.',
     navBreadcrumb: 'Breadcrumb',
+    navBreadcrumbHint: 'This is the path customers see above this category, e.g. Home › Electronics › Phones. It comes from the parent category you choose.',
+    iconHint: 'A small emoji or symbol shown next to the category name in menus, e.g. 📱. Leave empty for none.',
     navRoot: 'This is a top-level category.',
     navChildren: 'Subcategories',
     navNoChildren: 'No subcategories yet.',