fix(iOS): clear default NukeUI placeholder background

[Navipro70]

Summary

NukeUI's LazyImageView creates a default placeholderView with secondarySystemBackground color (#1c1c1e in dark mode).

This prevents users from customizing the background color via React Native styles, as the default placeholder covers the entire view before the image loads.

Changes

Setting placeholderView to nil in init allows the parent view's backgroundColor (set via RN style prop) to be visible.

override init(frame: CGRect) {
  super.init(frame: frame)
  addSubview(lazyImageView)
  // Clear the default NukeUI placeholder (has secondarySystemBackground color)
  lazyImageView.placeholderView = nil
  ...
}

Notes

• This does not affect custom placeholders (blurhash, thumbhash, memoryCacheKey) - they still work as expected
• This does not affect the indicator prop - it still works as expected
• Users can now control background color via the style prop

Fixes #405

[duguyihou]

This looks great and definitely improves the project. I’ve just merged it. Appreciate your help!