feat: Add backgroundImageWidget property for custom background images

[remax21]

What

Adds a new backgroundImageWidget property to PageViewModel that accepts any widget as a background image.

Why

The existing backgroundImage parameter uses hardcoded values for BoxFit, width, height, and alignment. This new property gives full control over image properties.

Usage

PageViewModel(
  backgroundImageWidget: Image.asset(
    'assets/background.jpg',
    fit: BoxFit.contain,
    width: 300,
    alignment: Alignment.topCenter,
  ),
  // ...
)