test: add PriceSparkline component and unit tests for data point rendering - #576

Summary

Adds the missing PriceSparkline SVG component and comprehensive unit tests for rendering the correct number of data points, edge cases, and line color.

What changed

• src/components/common/PriceSparkline.tsx — New component that accepts a dataPoints: number[] prop and renders an SVG <path> polyline. Handles 7+ points (full line), 1 point (circle dot, no path), and 0 points (returns null).
• src/components/common/__tests__/PriceSparkline.test.tsx — Co-located tests covering all acceptance criteria.

Key design decisions

• Component is standalone and agnostic about data source — accepts dataPoints directly rather than coupling to the Course model (which has no priceHistory field). Integration into CreatorCard is a follow-up.
• Uses emerald-400 (#34d399) for the positive line color, matching the design system already used in RecentActivityBadge, Change24hBadge, etc.
• For 0 data points → returns null so callers see nothing. For 1 data point → renders an SVG with a dot (no line path), which satisfies "no error" without visual breakage.

Acceptance criteria checklist

• 7 data points produce a rendered <path> element
• 1 data point renders without throwing (no line path, dot only)
• 0 data points renders nothing (empty container)
• Green line colour (#34d399) applied when last value > first value

Test output

✓ src/components/common/__tests__/PriceSparkline.test.tsx (4 tests) 79ms
  Tests  4 passed (4)

Follow-ups

• Wire PriceSparkline into CreatorCard.tsx when the API provides priceHistory data
• Add a red/rose line color for negative trends (not in scope of this issue)

Security note

No secrets, user input, or auth tokens are involved. The component only renders static SVG from numeric data.

Closes #572