feat(admin): Shopify-style variant attributes matching production data shape

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

P0 user feedback: variants were just free-text name+price, and the user shared the real production payload - a flat array of {color, size, price, currency, remaining} rows, colors as '0x8B4513' hex.

- New model: AdminProductVariantAttributeDef (key/label/isColor/values) + AdminProductVariant (attributes: Record<string,string>, sku, image, remaining, prices: {currency,price}[]) - this IS the production shape, grouped by combo with one row per currency instead of flattened, so admins edit one variant card instead of 4 duplicate rows
- Attribute manager: add custom attributes (Color, Size, or anything), color attributes get a native color picker + live swatch preview instead of typing hex; other attributes get plain value chips
- 'Generate variants' computes the cartesian product of attribute values (Color x Size = 4 combos for 1 color x 4 sizes) and preserves existing sku/price/stock data for combos that still exist after regeneration
- Multi-currency pricing per variant (matches prod: same combo priced in RUB/USD/EUR/AMD) with per-currency add/remove
- Color hex kept in the exact '0x8B4513' production format (toBackendColor/toCssColor conversion helpers)
- Fixed an Angular v21 control-flow parser bug hit while building this: an @if/@else block whose only content is a bare {{ interpolation }} touching the block's closing brace fails to parse (NG5002 'Unclosed block for' cascading from a completely unrelated line) - worked around by keeping interpolation in its own element
- Verified end-to-end in browser: added Color (color picker) + Size (S/M/L/XL) attributes, generated 4 variant combos, added all 4 currencies to a variant - matches the shared production JSON exactly

src/app/i18n/en.ts

@@ -1554,6 +1554,20 @@ export const en: Translations = {
     variantName: 'Variant name',
     variantsHint: 'Each variant is a purchasable option of this product, e.g. a size or color, with its own price and stock.',
     variants: 'Variants',
+    variantAttributes: 'Options',
+    variantAttributesHint: 'Add options like Color or Size, then list the values customers can choose from. Combinations below are generated automatically.',
+    colorAttribute: 'Color',
+    removeAttribute: 'Remove option',
+    removeValue: 'Remove value',
+    pickColor: 'Pick a color',
+    newValuePlaceholder: 'e.g. Small',
+    addValue: 'Add',
+    newAttributePlaceholder: 'e.g. Size',
+    addAttribute: 'Add option',
+    generateVariants: 'Generate variants',
+    variantsList: 'Variants',
+    remaining: 'Stock',
+    variantImage: 'Image',
     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.',