Standardize panel borders across modules using theme divider

[Irozuku]

Summary

Standardized panel border colors across the application by updating shared layout components to use the theme’s divider color. This aligns the visual styling of all module content areas with the Home page.

Before

After

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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)

• components/layout/ThreeSectionLayout.jsx: Updated shared layout containers (SideBar and CenterBox) to use theme.palette.divider for borders.
• components/notebook/LeftPanel.jsx: Removed hardcoded border styling to rely on shared theme-based styling.
• components/notebook/RightPanel.jsx: Removed redundant border styles for cleaner and more consistent panel appearance.
• components/datasets/DataloaderConfigBar.jsx: Refactored to use the shared SideBar component and updated header border styling to use the theme divider color.

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Testing (optional)

• Verify that panel borders across all modules match the Home page styling.
• Confirm border appearance remains consistent in both light and dark themes.
• Ensure notebook panels, sidebars, and dataloader configuration panels render correctly without layout regressions.

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Notes (optional)

This change is purely visual and focuses on unifying the application's panel styling by centralizing border definitions in shared layout components.