feat: Implement sample addition to packs functionality

Commit notes

This commit introduces the ability to add samples to existing sample packs through a new API endpoint and corresponding frontend functionality. The backend now supports a POST request to add samples, ensuring that only samples owned by the user can be added. The frontend includes a new mutation hook for adding samples and updates the library page to allow users to select and drag samples into packs.

New features: - Backend endpoint for adding samples to packs - Frontend mutation hook for adding samples - UI enhancements for sample selection and drag-and-drop functionality