Define disabled elements via opacity rather than color

[ariser]

Having a “disabled” text color is not the best approach. A disabled color varies depending on where an item is displayed. And as a rule, “disabled” is not a color, disabled items have low contrast. So it's easier and more convenient to use opacity rather than color.

Suggested change

• review disabled states of click-ui components
• for text, stop using disabled color, and use opacity instead
• for backgrounds and other visual attributes, review each case. Use opacity where possible, otherwise keep the color
• review the changes in both light and dark themes

Originally posted by @ariser in https://github.com/ClickHouse/click-ui/issues/610#issuecomment-2980903330