feat: replace blue accent with amber/gold color scheme
- Swap sapphire blue palette for warm amber/gold (#f3bc00)
- Add complete amber color scale (50-900) for light/dark mode harmony
- Update semantic tokens: --accent, --accent-hover, --accent-subtle, --accent-muted
- Fix text contrast: dark brown (#3f2e00) on amber backgrounds
- Use lighter amber (#fbbf24) for dark mode visibility
- Add amber-to-orange gradient for .gradient-text utility