Update Go module dependencies and enhance avatar upload functionality

Commit notes

- Added new dependencies for image processing: `github.com/chai2010/webp` and `github.com/disintegration/imaging` to support WebP image encoding and manipulation. - Enhanced the `UploadAvatar` method in the `AuthHandler` to handle multipart avatar uploads, generate WebP variants, and store them in the storage system. - Updated the `SamplePackHandler` and `MarketHandler` to include cover image URL generation for different sizes, improving the handling of cover images in sample packs. - Refactored various tests to accommodate changes in token generation and avatar handling, ensuring consistent behavior across the application.