fix: improve nudge contrast in chat

[stablegenius49]

Summary

• switch chat nudges from text-placeholder-foreground to text-muted-foreground for stronger contrast in both themes
• use the accent hover background so the nudge hover state is more visible

Verification

• cd frontend && npx -y @biomejs/biome lint app/chat/_components/nudges.tsx
• checked contrast ratios from the theme tokens used by this component:
  • light mode: 2.55:1 → 4.86:1
  • dark mode: 3.65:1 → 6.94:1

Fixes #796