Define disabled elements via opacity rather than color

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_