feat(#214): toggle chart series via legend

[Hollujay]

Summary

Adds support for toggling chart series on/off by clicking legend items in the Chart component. The chart now accepts a series prop alongside the existing data prop for backward compatibility.

Changes

src/components/Chart.jsx

• Added series prop — accepts an array of { name, color, data } objects for multi-series grouped bar charts
• When series is provided, a clickable legend is rendered below the chart
• Clicking a legend item toggles that series visibility (dimmed bars, reduced opacity)
• Scale recalculates based on visible series only
• Tooltips show series name in multi-series mode
• Existing data prop continues to work unchanged

src/components/Chart.css

• Added .chart-legend, .chart-legend-item, .chart-legend-item--dimmed, .chart-legend-swatch, .chart-legend-label styles

src/components/Chart.stories.jsx

• Added MultiSeries story showcasing two series (Sent vs Received) with legend

test/components/Chart.test.jsx

• Added 6 new tests covering:
  • Legend renders with series names
  • Correct bar count for multi-series
  • Series visibility toggled on legend click
  • Legend item dims when series is hidden
  • Tooltip shows series name on hover
  • Default colors assigned when omitted

Testing

All 13 tests (7 existing + 6 new) pass.

Closes #214

[arisu6804]

Tidy PR, thanks! ✅