feat: improve terminal text readability with better background detection

[andymccutcheon]

Summary

Improves terminal text readability by enhancing background detection
and using brighter colors on dark terminal backgrounds.

Problem

Dark blue text in the setup wizard was nearly impossible to read on
dark terminal backgrounds, particularly affecting labels like "Selected
endpoint:", "User PII Access:", etc.

Solution

1. Enhanced Terminal Detection:

   • Improved isDarkBackground() function with better heuristics
   • Added support for NO_COLOR environment variable
   • Added detection for Hyper and Warp terminals
   • Changed fallback to assume LIGHT backgrounds (safer default)

2. Improved Color Contrast:

   • Link colors: sky-300 (#7DD3FC) on dark, sky-700 (#0369A1) on light
   • Muted text: gray-200 (#E5E7EB) on dark, gray-700 (#374151) on
     light
   • Applied consistent colors across all UI functions

3. Updated Functions:

   • formatKeyValue() - Key-value pair formatting
   • showSection() - Section headers
   • showCompletion() - Completion messages
   • showDivider() - Dividers
   • formatKeychainChoiceLabel() - Key selection lists

Testing

• ✅ All 201 unit tests pass
• ✅ Build succeeds with no errors
• ✅ Linting and formatting checks pass
• ✅ Manually tested with both light and dark terminal themes