feat(marketplace): adjust cart and checkout view to marketplace use-case (MS-1158)

[GrzegorzDerdak]

I want to merge this change because it adjusts the storefront cart and checkout flows to support marketplace multi-vendor purchases.

This PR introduces a marketplace-aware checkout cookie format, where checkout IDs are stored per vendor, and updates the cart, PDP, checkout, payment, and order summary flows to work with multiple vendor checkouts.

Related issue: MS-1158

Testing process and results

• Verify adding products from different vendors creates separate vendor checkout entries.
• Verify cart groups products by vendor and shows the correct vendor names.
• Verify deleting the last line from a vendor cart removes that vendor checkout from the cookie.
• Verify standard cart and checkout still work when marketplace mode is disabled.
• Verify checkout steps work for marketplace orders: user details, shipping address, delivery method, payment.
• Verify locale switching clears the checkout cookie.
• Run checkout E2E tests for guest and logged-in flows.

Impact

Buyers can now use a multi-vendor cart and checkout flow where products are grouped by seller, delivery methods are selected per vendor checkout, and marketplace payment can be initialized from multiple checkouts.

The existing single-vendor storefront flow continues to use shared checkout helpers and should keep working with the new cookie format. Legacy string checkout cookies are handled by fallback parsing.

Rollback plan

Revert this PR to restore the previous single-checkout cart and checkout behavior. No data migration should be required because checkout state is stored in cookies and the new helpers keep legacy cookie fallback support.

Pull Request Checklist

• Test the changes locally to ensure they work as expected.
• Document the testing process and results in the pull request description. (Screen recording, screenshot etc)
• Include new tests for any new functionality or significant changes.
• Ensure that tests cover edge cases and potential failure points.
• Document the impact of the changes on the system, including potential risks and benefits.
• Provide a rollback plan in case the changes introduce critical issues.
• Update documentation to reflect any changes in functionality.

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Project	Deployment	Actions	Updated (UTC)
nimara-ecommerce	Ready	Preview, Comment	May 7, 2026 10:07am
nimara-marketplace	Ready	Preview	May 7, 2026 10:07am

2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
nimara-docs	Skipped		May 7, 2026 10:07am
nimara-ecommerce-stripe	Skipped		May 7, 2026 10:07am