[Swift][Demo]: Add a checkout presentation picker

[kieran-osgood-shopify]

What changes are you making?

This is a pre-emptive change to support testing of #649.

Add a persisted Checkout presentation picker under Settings → Features in the Swift demo.

The picker changes the mechanism we use to present checkout

SwiftUI (default)	UIKit
Existing ShopifyCheckout sheet	Direct CheckoutViewController construction and UIKit presentation

Closed	Open

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Before you merge

Important

• I've added tests to support my implementation
• I have read and agree with the Contribution Guidelines
• I have read and agree with the Code of Conduct
• I've updated the relevant platform README (platforms/swift/README.md and/or platforms/android/README.md)

SDK README changes are not required for this sample-only setting.

[github-actions]

Bundle Size Budgets

Budget	Size	Limits	Result
—	—	—	➖ No configured budgets affected

Bundle and package size

Web bundle sizes cover shipped runtime JavaScript. Package sizes cover the full published archive, including any source maps, declarations, and documentation it contains.

Platform	Measurement	Compression	Base	Head	Delta
-	-	-	-	-	-

How sizes are measured

Measured from the PR base SHA and PR head SHA. Web bundle rows sum shipped .js, .mjs, and .cjs files under dist/, excluding source maps and declarations. The gzip bundle size sums files compressed individually with gzip -n -9. npm package sizes are gzip-compressed .tgz archives; Android AAR sizes are ZIP archives. Package sizes are not final app binary sizes.

[markmur]

Nice ✨

[bitrise]

Bitrise builds

E2E · iOS CI

Install this build

Open Tophat, select your target device, then click Install. Links open on the Mac running Tophat.

SDK	Install
Swift	Install with Tophat

Checkout Kit E2E results

Status	Tags	Target	Platform	OS version tag	Device
✅	launch, checkout-presentation, checkout-completion, buyer-identity, preload	swift	ios	latest	iPhone 15 iOS 27 Beta