feat(market-card): add prefers-contrast: more support with explicit border+text overrides (Closes #772)

[waterWang]

Summary

Adds prefers-contrast: more (OS-level High Contrast mode) support to the MarketCard component with explicit border and text overrides.

Changes

• app/styles/themes/market-card-contrast.css (new): A @media (prefers-contrast: more) block that applies:
  • Solid 2px white border on the card
  • Higher-contrast text colors (gray-100/200/300 instead of white/70/60)
  • Explicit green-400/red-400 for odds values
  • Stronger probability bar background
  • Visible border on following indicator
  • Disabled backdrop-blur (which can reduce contrast)
• app/components/MarketCard.tsx: Added market-card CSS class and imported the contrast CSS file.
• app/components/__tests__/MarketCard.contrast.test.tsx (new): 3 tests verifying the market-card class is applied correctly.

Testing

npx jest MarketCard.contrast --no-cache
# PASS — 3 tests

Closes #772

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