fix(ui): honor includeBackground for non-animated decorator backgrounds

[kar]

Regression

Since 4.0.0, AdaptyUIAnimatableDecoratorModifier.bodyWithBackground applies the includeBackground opacity gate only in the animated-background branch; the static branch paints the decorator background unconditionally:

if let animatedBackgroundFilling {
    content.background {
        self.backgroundFill(for: animatedBackgroundFilling)
            .opacity(includeBackground ? 1.0 : 0.0)   // gated
    }
} else if let background = self.decorator.background {
    content.background {
        self.backgroundFill(for: background)          // NOT gated
    }
}

In 3.17.x every branch was gated (AdaptyUIDecoratorModifier, all three branches applied .opacity(includeBackground ? 1.0 : 0.0)).

Visible effect

The footer is the only element rendered with includeBackground == false (drawDecoratorBackground in AdaptyUIHeroContainerView / AdaptyUIFlatContainerView): its background is meant to appear only once scrollable content extends beneath the footer. With the gate lost, a translucent footer background (e.g. #000000BF in dark mode) is composited over the screen content unconditionally, visibly darkening the footer band on any paywall using one.

Measured on a production paywall (legacy Paywall Builder template basic, dark mode): footer band rgb(57,57,57) under 3.17.2 vs rgb(14,14,14) under 4.0.2 — exactly one extra application of the #000000BF scrim over the same backdrop.

Fix

Restore the gate on the static branch, matching the animated branch and the 3.x behavior. One line.