Enhance sample preview handling and UI components

Commit notes

This commit updates the PreviewSample method in the SampleHandler to allow owners to preview their unpublished samples, improving user experience. It also introduces a new Preview button in the PackManager component for easier access to sample previews. Additionally, the SampleTreeView component is modified to handle authenticated preview URLs, ensuring proper authorization. The PackUploader component is enhanced with drag-and-drop functionality for file uploads, improving usability. Lastly, the theme colors in root.tsx are updated for a refreshed visual design.