Add chart tooltip with formatted values (#211)

[Hollujay]

Summary

Adds an interactive tooltip to the chart component that displays formatted values when hovering over bars. The tooltip shows the numeric value (optionally formatted via a formatValue callback) and an optional label for each data point.

Changes

src/components/Chart.jsx

• Added hoveredIndex state to track which bar is being hovered
• Added formatValue prop — a function that receives a data item and returns a formatted string for the tooltip
• Added onMouseEnter/onMouseLeave handlers on each <rect> bar
• Hovered bar uses a darker shade (#4f46e5) for visual feedback
• Renders a role="tooltip" element centered above the hovered bar, with formatted value and optional label

src/components/Chart.css

• Added .chart-wrapper for absolute positioning context
• Added .chart-bar with pointer cursor and smooth fill transition
• Added .chart-tooltip positioned above the bars, styled consistently with the existing Tooltip.css
• Added .chart-tooltip-value (bold) and .chart-tooltip-label (muted, text-overflow ellipsis) sub-elements
• Added prefers-contrast: more media query override matching existing patterns

src/components/Chart.stories.jsx (new)

• Storybook stories for Default, WithFormattedValues, and SingleBar variants

src/pages/Transfers.jsx

• Passes formatValue={(d) => formatAmount(d.value, d.currency)} to format transfer amounts as currency
• Data items now include label (recipient) and currency for richer tooltip display

test/components/Chart.test.jsx

• 6 tests covering: title rendering, bar count, tooltip show/hide on hover, raw value display, formatted value display, and label display.

Closes #211

[arisu6804]

LGTM — clean work, merging! 🚀