fix: improve color contrast to meet WCAG AA standards (text-slate-500→text-slate-600 on bg-slate-50) [fj4WqyCCw3C5ShR1RfB7MoBPTpkRrBFYP1uT35g3MvT]

[waterWang]

Closes #395

Summary

Replaced text-slate-500 with text-slate-600 on bg-slate-50 backgrounds and dark:text-slate-500 with dark:text-slate-400 on bg-slate-950 backgrounds to improve WCAG AA color contrast compliance.

Changes

• page.tsx: Updated scroll-down hint text
• not-found.tsx: Updated 404 page description
• error.tsx: Updated error page message
• BountyDetailClient.tsx: Updated Owner/Reward/Status labels
• BountyListClient.tsx: Updated end-of-list message
• SavedBountiesList.tsx: Updated loading/empty/connect states
• dashboard/page.tsx: Updated empty state and table headers
• bounties/new/page.tsx: Updated preview placeholder text
• StatusBadge.spec.tsx: Added test coverage for badge rendering (6 tests)

Contrast improvements

Before	After	Ratio
text-slate-500 on bg-slate-50	text-slate-600 on bg-slate-50	4.0:1 → 7.2:1 ✅ AA
dark:text-slate-500 on bg-slate-950	dark:text-slate-400 on bg-slate-950	4.2:1 → 7.9:1 ✅ AA

Verification

• ✅ Build passes (npm run build)
• ✅ 6 new StatusBadge tests pass
• ✅ No visual regression (same font weight, size, and layout)