style: redesign bar chart hover highlighting in dataset tabs

[Creylay]

Summary

Replaced Recharts' default bar hover effect (bright background glow) with a cleaner highlight: the hovered bar becomes fully opaque while the rest drop to 50% opacity. No outline or external background is added.

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Type of Change

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

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Changes (by file)

• DashAI/front/src/components/notebooks/dataset/tabs/OverviewTab.jsx: added Cell components with dynamic fillOpacity on hover; disabled default active bar glow (activeBar={false}, cursor={false} on Tooltip).
• DashAI/front/src/components/notebooks/dataset/tabs/CategoricalTab.jsx: same hover logic; state keyed by column name to handle multiple charts on the same page.
• DashAI/front/src/components/notebooks/dataset/tabs/TextTab.jsx: same hover logic; all bars use a uniform fill color (#8884d8) to match the original appearance.

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Testing

• Open a dataset with missing values and verify the Missing Values Overview chart: hovering a bar dims the rest to 50% opacity with no background glow.
• Open a dataset with categorical columns and verify the Value Distribution chart behaves the same way.
• Open a dataset with text columns and verify the Length Distribution chart behaves the same way. Confirm all bars remain the same purple color (no per-bar color variation).
• Confirm the tooltip still appears on hover with the correct value in all three charts.