Rewrite task text in Colors (4 lessons) and Box Model (8 lessons x 6
locales) to describe visual outcomes instead of giving copy-paste CSS
answers. Colors validations changed from property_value to regex
accepting multiple valid named colors per lesson.
Replace copy-pasteable CSS declarations in all 6 flexbox lesson tasks with
outcome-oriented descriptions. Update validation error messages to hint at
properties without revealing exact declarations. Add regex validation for
flexbox-6 to accept both flex: 1 and flex-grow: 1.
