Add method to blend colors

[mikhailChelbaev]

Originally, if the component's background color was not provided, it was calculated as main.withOpacity(0.15). This caused an issue where underlying content in cards and modals could be visible when a transparent color was used. To address this, a second non-transparent background view with .background color was added.

To avoid having second views in these components, this pull request introduces a new .blend method that blends an original color with a provided color using the following formula:

red = self.red * self.alpha + other.red * (1.0 - self.alpha)
green = self.green * self.alpha + other.green * (1.0 - self.alpha)
blue = self.blue * self.alpha + other.blue * (1.0 - self.alpha)

This ensures that the resulting color's alpha is always 1.0.

The calculation for the component's background color has been updated to main.withOpacity(0.15).blend(with: .background), ensuring it is always opaque.

As a result, the secondary backgrounds from modals and cards have been removed.