feat(admin): collapse product translations behind default-language fields

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

P0 user feedback: four parallel description fields (default/ru/en/hy) read as duplicates and confused the admin.

- The default-language name/short description/rich description now stand alone; per-language fields moved into a collapsed 'Translations' disclosure with a translated-count badge (e.g. 1/3)
- The disclosure explains the fallback rule in merchant language: empty translation means customers see the default text
- Translation inputs show the default value as placeholder, making the fallback visible instead of implied
- Locale codes shown uppercase (RU/EN/HY) to read as language labels, not field suffixes

src/app/i18n/en.ts

@@ -1545,6 +1545,7 @@ export const en: Translations = {
     generate: 'Generate',
     seoGenerate: 'Fill from product details',
     seoGenerateHint: 'Fills empty fields from the product name and short description. Your existing text is kept.',
+    translationsHint: 'The main fields above are shown in your default store language. Add translations here for other languages - if a translation is empty, customers see the default text instead.',
     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.',