Plugins "Get started": fix blank page + duplicate site when backing out of checkout

[taipeicoder]

Proposed Changes

Fix backing out of checkout in the per-plugin "Get started" (with-plugin) flow, which had two separate failures:

• Blank page. The checkout back URL dropped the required billing_period query dependency for free plugins (which have no billing period), so the flow controller rejected the URL and the plans step rendered blank. It now always includes billing_period (empty for free plugins).
• Duplicate site. Re-entering the flow restarted at the domains step and created a second site. The framework already has a "skip the domains step on browser-back from checkout" path for the onboarding flow (persist domains data on completion, re-submit it on re-entry so the site is reused). This extends that path to the with-plugin flow.

Net result: backing out of checkout lands on the plans grid with the plugin selected — no blank page, no duplicate site.

Testing Instructions

1. Log out, open a free marketplace plugin (e.g. MailPoet) and a paid one (e.g. Sensei Pro), "Get started".
2. Create an account, pick a domain, land on the plans grid, pick a plan → checkout.
3. Click the checkout back control.
4. Confirm you return to the plans grid (not blank, not the domains step) and that no duplicate site was created.

Unit tests in client/signup/test/flows.js cover the back URL carrying billing_period for both paid and free plugins. The domains-skip behavior was verified manually for both plugin types.

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?
• For UI changes, have you tested the affected components in dark mode?
• Have you tested accessibility for your changes?
• Have you used memoizing on expensive computations?
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• 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]

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[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~46 bytes added 📈 [gzipped])

Details

name                      parsed_size           gzip_size
signup                         +117 B  (+0.0%)      +46 B  (+0.1%)
async-step-unified-plans        +60 B  (+0.0%)      +20 B  (+0.0%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.