feat: enhance vendor product edit form with all mandatory fields

- Add translations support with language tabs (EN, FR, DE, LU)
- Add product identifiers: vendor SKU with auto-generate, brand, GTIN, GTIN type
- Add pricing fields: price (incl. VAT), sale price, currency, VAT rate
- Add primary image field with preview
- Add product status (active, featured) checkboxes
- Add optional supplier info section
- Pre-populate form with existing product data from API
- Add form validation (isFormValid method)
- Make is_digital read-only (derived from marketplace product)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/schema/vendor_product.py

@@ -87,6 +87,9 @@ class VendorProductDetail(BaseModel):
     vendor_code: str | None = None
     marketplace_product_id: int
     vendor_sku: str | None = None
+    # Product identifiers
+    gtin: str | None = None
+    gtin_type: str | None = None  # ean13, ean8, upc, isbn, etc.
     # Product fields with source comparison
     price: float | None = None
     price_cents: int | None = None
@@ -163,20 +166,41 @@ class VendorProductCreate(BaseModel):
     description: str | None = None
 
 
+class TranslationUpdate(BaseModel):
+    """Translation data for a single language."""
+
+    title: str | None = None
+    description: str | None = None
+
+
 class VendorProductUpdate(BaseModel):
     """Schema for updating a vendor product."""
 
-    title: str | None = None
+    # Translations by language code (en, fr, de, lu)
+    translations: dict[str, TranslationUpdate] | None = None
+
+    # Product identifiers
     brand: str | None = None
     vendor_sku: str | None = None
     gtin: str | None = None
-    price_override: float | None = None
-    currency_override: str | None = None
-    availability: str | None = None
+    gtin_type: str | None = None  # ean13, ean8, upc, isbn, etc.
+
+    # Pricing
+    price: float | None = None  # Price incl. VAT in euros
+    sale_price: float | None = None  # Optional sale price
+    currency: str | None = None
+    tax_rate_percent: int | None = None  # 3, 8, 14, 17
+
+    # Status (is_digital is derived from marketplace product, not editable)
     is_active: bool | None = None
     is_featured: bool | None = None
-    is_digital: bool | None = None
-    description: str | None = None
+
+    # Images
+    primary_image_url: str | None = None
+
+    # Optional supplier info
+    supplier: str | None = None
+    cost: float | None = None  # Cost in euros
 
 
 class VendorProductCreateResponse(BaseModel):