feat: add Checkout Components example gallery

[OnurVar]

Summary

Replaces the single-row Checkout Components screen in the example app with an 8-example gallery, ordered as the customization ladder and badged per tier:

1. Drop-in checkout — prebuilt sheet, zero custom UI
2. Themed drop-in — same sheet re-branded with one theme prop
3. eSIM marketplace — composed page: method list, inline card form, dynamic pay button
4. Coffee reorder — one-tap repurchase on usePrimerVaultManager, new-card fallback
5. Accordion checkout — payment embedded in a merchant flow with expand/collapse sections
6. Coin top-up — compact card form inside a popup dialog, outcome rendered in-dialog
7. Card preview — live card mirroring form state, co-badge pick reflected on the card face, slide-to-pay gesture
8. Bring-your-own UI — plain TextInputs wired to the hooks only, custom network picker, no Primer components

The list screen fetches a client session per example and presents overlays in place or pushes screens with the token as a route param. Every screen handles loading, processing, success and error (processing mirrors the drop-in's pattern: flag from submit/retry initiation until the outcome lands).

Example-app only; no SDK changes.

Merge gate

Merges after its parents; same ADR gate: https://app.notion.com/p/377ca65dc30e81de958bdc3bb2b887b8

Stacked on

#381 (naming) → #382 (context provider) → this.

Jira

https://primerapi.atlassian.net/browse/ORC-6924

Test plan

• example app typecheck at its pre-existing baseline
• manual: each row end-to-end with the sandbox Mastercard; co-badge picker via 4035 5010 0000 0008 on rows 7–8

[github-actions]

	Warnings
⚠️	This PR doesn't seem to contain any updated Unit Test for Swift 🤔. Please consider double checking it 🙏

	Messages
📖	✅ No SwiftLint violations found.

Generated by 🚫 Danger Swift against 7d9f4c5

[unblocked]

1 issue found.

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[unblocked]

The pay button is disabled only when !payEnabled, but it does not account for showProcessing. While the processing overlay covers the screen, a fast tap before the overlay renders (or on slower devices) can trigger handlePay a second time since the button is still technically enabled. Other screens in this PR (e.g. AccordionCheckoutScreen line 352, CoffeeReorderScreen line 422) correctly use disabled={!payEnabled || showProcessing}.

Suggested change

	<TouchableOpacity
	style={[styles.payButton, !payEnabled && styles.payButtonDisabled]}
	onPress={handlePay}
	disabled={!payEnabled}
	>
	disabled={!payEnabled || showProcessing}

[github-actions]

	Warnings
⚠️	This PR doesn't seem to contain any updated Unit Test for Kotlin 🤔. Please consider double checking it 🙏

	Messages
📖	✅ No detekt violations found.

Generated by 🚫 Danger Kotlin against 7d9f4c5

[github-actions]

	Warnings
⚠️	Pull Request size seems relatively large. If this Pull Request contains multiple changes, please split each into separate PRs for a faster, easier review.
⚠️	Please assign someone aside from CODEOWNERS (@checkout-pci-reviewers) to review this PR.

	Messages
📖	✅ No ESLint violations found.

Generated by 🚫 dangerJS against 7d9f4c5