fix: improve contrast for gradient actions

[efegokdemir]

Summary

Fixes #149 by giving interactive gradient actions darker stops that preserve the existing brand gradient while meeting WCAG AA contrast for white text.

Changes

• Add a dedicated --gradient-action token with contrast-safe violet, magenta, and amber stops.
• Add the bg-gradient-action utility.
• Apply the accessible gradient to authentication, navigation, marketplace, ticket, verification, dashboard, and pill action controls.
• Keep the original bg-gradient-sunset for decorative and non-interactive brand surfaces.

Testing

• npm run lint
• npx tsc --noEmit
• npm test -- --run (5 files, 20 tests passed)
• npm run build
• git diff --check
• Contrast sampled across the gradient: white text remains at least 4.5:1 at all stops and sampled intermediate positions.

Notes

The original brand gradient remains unchanged for decorative uses; interactive controls use a separate token so the contrast fix does not alter the broader visual identity.

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[efegokdemir]

Rebased onto latest main and resolved the contrast conflict by carrying the action-gradient change into the current shared Button variant. Validation: git diff --check; TypeScript validation is blocked by an existing syntax error in src/app/my-tickets/page.tsx.