feat: Allow custom transition animation for popups

[videni]

Summary

Add support for custom transition animations on vertical popups (TopPopup/BottomPopup).

Closes #199

Changes

• Add transition property to LocalConfigVertical
• Add transition(_ value: AnyTransition) configuration method
• Update PopupVerticalStackView to use custom transition when configured
• Default behavior unchanged (.move(edge:) transition)

Usage

func configurePopup(config: BottomPopupConfig) -> BottomPopupConfig {
    config
        .transition(.opacity)  // Fade in/out instead of slide
}

// Or combined transitions
func configurePopup(config: TopPopupConfig) -> TopPopupConfig {
    config
        .transition(.scale.combined(with: .opacity))
}