feat: Introduce UpdateVariantRequest DTO for updating product variants

[gustaavik]

This pull request introduces several updates to enhance the handling of product variants across the backend and frontend. The changes include the addition of a new UpdateVariantRequest structure, updates to request parameter handling, and improvements to error handling for variant-related operations.

Backend Changes

Data Structures:

• Added UpdateVariantRequest struct to represent the data needed to update an existing product variant in internal/dto/product.go. This includes fields for SKU, Price, Stock, Attributes, Images, and IsDefault.

API Endpoint Logic:

• Updated UpdateVariant and DeleteVariant methods in internal/interfaces/api/handler/product_handler.go to use productId and variantId instead of id and variant_id for URL parameters. [1] [2] [3] [4]
• Replaced the use of CreateVariantRequest with UpdateVariantRequest in the UpdateVariant method and added conditional checks to handle optional fields (Price, Stock, IsDefault). [1] [2]
• Improved error handling in AddVariant, UpdateVariant, and DeleteVariant methods by removing seller-specific authorization checks and refining error messages for better clarity. [1] [2] [3]

Frontend Changes

Type Definitions:

• Added UpdateVariantRequest interface in web/types/api.ts to mirror the backend structure for updating product variants. This includes fields for sku, price, stock, attributes, images, and is_default.