Fix dataset undefined values and selection hover bug

[Irozuku]

Summary

This pull request improves user interface feedback and strengthens dataset data validation.

It fixes an issue where newly uploaded datasets displayed undefined values due to missing total_rows and total_columns during dataset merging. Additionally, it improves selection highlighting behavior so that selected items keep their background color when hovered, ensuring consistent visual feedback.

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Type of Change

Check all that apply like this [x]:

• Backend change
• Frontend change
• CI / Workflow change
• Build / Packaging change
• Bug fix
• Documentation

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Changes (by file)

• ItemBox.jsx: Updated hover background color logic to preserve the selected color when an item is hovered while selected; otherwise, the hover color is applied.
• useDatasets.js: Added validation checks to ensure total_rows and total_columns exist before merging dataset information, preventing undefined values for newly uploaded datasets.

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Testing (optional)

• Upload a new dataset and verify that total_rows and total_columns are displayed correctly (no undefined values).
• Select an item and hover over it to confirm the background color remains consistent while selected.