feat: add custom cover image support for datasets

[xuang7]

What changes were proposed in this PR?

This PR adds support for dataset owners to set a custom cover image from files within their dataset. This improves visual presentation and makes datasets easier to recognize on the hub/home page.

Changes:

• Database:

  • Added a cover_image column to the dataset table.
  • Stores cover image as {dvid}/{relative_path} format (e.g., v1/demo.png)

• Backend:

  • POST /api/dataset/{did}/update/cover – Set cover image
  • Validates the cover image size is below coverSizeLimit (default: 10 MB)

• Frontend:

  • Adds a “Set as Cover” button next to image files in the version file tree, button only visible for image file (.jpg, .jpeg, .png, .gif, .webp)
    
  • The hub/home page loads cover images from /api/dataset/file using the stored cover_image path.

Public datasets display:

Before (default cover)	After (customized cover)

Any related issues, documentation, discussions?

Closes #4115

How was this PR tested?

Tested manually.

Was this PR authored or co-authored using generative AI tooling?

Generated-by: Claude for code review

[chenlica]

@aicam please review it first before @aglinxinyuan does it.