Fix gradient overlay to cover full viewport instead of image bounds

[facumenzella]

Summary

Fixes gradient overlays on background images appearing more aggressive on Android compared to the web builder.

Problem

When a paywall has a background image with a color overlay (gradient), the overlay was being applied via the underlay() modifier which draws within the image's bounds. Since images are often aspect-fit and smaller than the viewport, a gradient defined as "66% white from bottom" would compress into that smaller area, making much more of the screen appear white.

Web builder: Overlay covers 100% of viewport → gradient spread across full screen
Android (before): Overlay only covered image bounds → gradient compressed into smaller area

Solution

Background.kt

• Removed colorOverlay application from the image background modifier
• Added alignment = TopCenter to position the image at the top

ViewWithVideoBackground.kt

• Extended WithOptionalVideoBackground to also handle BackgroundStyle.Image with overlays
• Renders overlay as a separate Box layer using .matchParentSize() to fill the full container
• Overlay renders between background image and content (correct Z-order)

Android	Dashboard