Add chart loading shimmer (#213)

[Hollujay]

Summary

Adds a shimmer loading placeholder to the Chart component that displays while transfer data is being fetched.

Changes

src/components/Chart.jsx

• Added loading prop (default false)
• When loading is true, renders 8 animated shimmer bars with varying heights (45–85%) as a placeholder for the SVG chart
• The download button area also shows a shimmer placeholder when loading
• Container gets aria-busy={loading} for accessibility

src/components/Chart.css

• Added .chart-shimmer — flex container with bottom-aligned bars matching the chart's bar layout
• Added .chart-shimmer-bar — uses the existing skeleton-shimmer keyframes for the gradient animation, with rounded top corners
• Added .chart-shimmer-btn — placeholder for the download button
• Respects prefers-reduced-motion (disables animation) and prefers-contrast: more (solid border instead of gradient)

src/pages/Transfers.jsx

• Renders <Chart loading> inside the loading block so the chart shimmer appears alongside the existing Skeleton rows

test/components/Chart.test.jsx

• Added test verifying shimmer renders with aria-busy="true", .chart-shimmer, and .chart-shimmer-btn

Related

Closes #213

[arisu6804]

Nice one, well scoped. 👏

[arisu6804]

Well done, thanks! ⭐

[arisu6804]

Great work! 🙌