Use Next.js/Vercel standard colors for error pages
Update error page colors to match the Next.js design system:
- Red: #e5484d (--color-red-700), dark mode: #ff6369
- Gray text: #666666 (--color-gray-900), dark mode: #a0a0a0
- Background: #fff / #0a0a0a
- Text: #171717 / #ededed
Also fix tests to use browser.eval() instead of browser.html()
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
