[MBL-2924] Design Polish - Navigation Buttons

[fernandacerezog]

📲 What

We are updating the close button in the navigation to standardize the visual experience and ensure that the application has a polished look.

🤔 Why

The styles of the navigation buttons have been adjusted to ensure visual consistency. The appearance has been updated to make the component look integrated.

🛠 How

1. SwiftUI view: CloseButtonView (SwiftUI) was created, which applies .glassedEffect(in: Circle(), interactive: true) to the close button.
2. Host controller: The SwiftUI view was integrated using UIHostingController, and navigationCloseButton returns the host controller view using guard let instead of force unwrap.
3. Extensions for Liquid Glass: Backward-compatible extensions were added:

• iOS 26+ Native API: The glassEffect() modifier (iOS 26+) is SwiftUI-exclusive with no UIKit equivalent. Migrating to UIKit would force us to use a fallback for all versions, losing the native iOS 26+ liquid glass effect.
• Code Organization: We've addressed organization concerns by extracting this component and the glassedEffect helper into separate files (SwiftUIHelpers/), keeping UIKit files clean.
• Future-Ready: Keeping this in SwiftUI allows us to leverage iOS-exclusive SwiftUI features while maintaining backward compatibility through our fallback implementation.

👀 See

| Before 🐛 | After 🦋 |
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✅ Acceptance criteria

• All navigation buttons match the design standard.
• No visual regressions in navigation bars.

[amy-at-kickstarter]

While we do write new code in SwiftUI, our team standards are to move views over at the level of an entire feature or entire screen. Because this is a single button in an existing UIKit view, it should be written in UIKit.

I would also recommend moving the glassedEffect into its own helper file, since it looks like you'll be using it in more than one place.

[amy-at-kickstarter]

I don't see Combine being used, this import should be deleted.

[amy-at-kickstarter]

This view is very small. Because it is a small component of a UIKit view, it should be written in UIKit.