feat(admin): product editor auto-slug, SKU helper, one-click SEO fill

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

P0 user feedback: admins did not know what SKU means, had to hand-write slugs, and left SEO empty.

- Name edits now derive the URL slug automatically (supports Cyrillic/Armenian characters); the derivation stops the moment the slug no longer matches the auto value, so a manually edited slug is never overwritten - verified in browser
- SEO title mirrors the name under the same only-while-untouched rule
- SKU field explains itself (what a stock keeping unit is, that any unique text works) and gains a Generate button producing readable codes like WIR-GAM-7K2P from the product name
- SEO tab gains 'Fill from product details': fills only empty title/description/keywords from name and short description, existing text untouched
- New adminProducts keys (slugHint, skuHint, generate, seoGenerate, seoGenerateHint) in en/ru/hy

src/app/i18n/en.ts

@@ -1538,6 +1538,11 @@ export const en: Translations = {
     variantsHint: 'Each variant is a purchasable option of this product, e.g. a size or color, with its own price and stock.',
     variants: 'Variants',
     archived: 'Archived',
+    slugHint: 'The web address of this product. Filled in automatically from the name - edit only if you need a custom URL.',
+    skuHint: 'SKU (stock keeping unit) is your internal product code, used for inventory and order lookups. Any unique text works.',
+    generate: 'Generate',
+    seoGenerate: 'Fill from product details',
+    seoGenerateHint: 'Fills empty fields from the product name and short description. Your existing text is kept.',
     seoExplain: 'This is what shows up in search results — a clear title and description help customers find this product.',
     searchTitle: 'Search title',
     searchTitleHint: 'The headline shown in search results. Keep it short and descriptive.',