feat(cli): show setting descriptions in settings dialog

[afarber]

Summary

Display the description of the currently selected setting in the settings dialog. The descriptions were already defined in the schema but never shown to users.

Details

• Added a text area below the settings list that shows the description of the active setting
• Adjusted height calculations to account for the new description area
• Descriptions update as user navigates through settings with arrow keys
• Added tests to verify description displays and updates on navigation

Related Issues

Fixes #14394

How to Validate

1. Run npm run build && npm start
2. Type /settings to open the settings dialog
3. Navigate up/down with arrow keys
4. Verify the description text changes for each setting
5. Test with search (/) to confirm descriptions work for filtered results

Pre-Merge Checklist

• [ ] Updated relevant documentation and README (if needed)
• [x] Added/updated tests (if needed)
• [ ] Noted breaking changes (if any)
• [x] Validated on required platforms/methods:
  • [x] MacOS
    • [x] npm run
    • [ ] npx
    • [ ] Docker
    • [ ] Podman
    • [ ] Seatbelt
  • [ ] Windows
    • [ ] npm run
    • [ ] npx
    • [ ] Docker
  • [ ] Linux
    • [ ] npm run
    • [ ] npx
    • [ ] Docker