MSD: correctly go back to previous steps when cancelling /setup/domain's checkout

[fushar]

Part of DOTMSD-1126 -- fixing back button behavior

Proposed Changes

Pass and forward back_to param to all checkout URLs constructed by the domain flow (/setup/domain), so that canceling checkout will land to that back_to URL.

Testing Instructions

MSD - dotcom

1. Click Dashboard (dotcom) Live link.
2. Navigate to a site domains page (/sites/:siteSlug/domains).
3. Click "Add domain name" → "Search domain names".
4. Select a domain and proceed to plan selection and then checkout.
5. Click the back/cancel button in checkout.
6. Verify you land on /setup/domain.
7. Click the back button in the top bar.
8. Verify you return to the original site domains page.

MSD - CIAB

1. Click Dashboard (CIAB) Live link.
2. Navigate to a site domains page (/sites/:siteSlug/domains).
3. Click "Add domain name" → "Search domain names".
4. Select a domain and proceed to checkout.
5. Click the back/cancel button in checkout.
6. Verify you land on /setup/domain.
7. Click the back button in the top bar.
8. Verify you return to the original site domains page.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you tested accessibility for your changes? Ensure the feature remains usable with various user agents (e.g., browsers), interfaces (e.g., keyboard navigation), and assistive technologies (e.g., screen readers) (PCYsg-S3g-p2).
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
  • For UI changes, have we tested the change in various languages (for example, ES, PT, FR, or DE)? The length of text and words vary significantly between languages.
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-aUh-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-172629

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-172629&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-172629&env=a8c-for-agencies

Dashboard Live (dotcom) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-172629&env=dashboard

Dashboard Live (CIAB) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-172629&env=dashboard-ciab

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• blaze-dashboard
• help-center
• odyssey-stats

To test WordPress.com changes, run install-plugin.sh $pluginSlug msd-domain-back-checkout on your sandbox.

[StevenDufresne]

I end up back in the cart with a free site:

Screen.Recording.on.2026-03-20.at.09-23-38.mp4

[fushar]

Ohhh it's because from MSD-dotcom, links to stepper screens still go to [wpcalypso.]wordpress.com (as shown in your screenshot) 🙈 (p1773958869364639/1773786780.552529-slack-CRWCHQGUB)

To test on dotcom, you need to test locally, or manually replace wpcalypso.wordpress.com with the container hostname manually.

[StevenDufresne]

Working. 👍