Signup: preserve the ref parameter from login through account creation

[bluefuton]

Proposed Changes

• getSignupUrl() now carries a ref query parameter from the login page through to /start, /start/account, and /start/<signup_flow>.
• createAccount() forwards that ref to /users/new and /users/social/new, so the referrer is recorded against the new account.
• ref is only forwarded when it is a non-empty string after trimming; anything else (arrays, objects, whitespace) is dropped.
• Unit tests cover the new passthrough and the rejected values.

Why are these changes being made?

Sign-ups that start on akismet.com arrive at WordPress.com with ?ref=…, but the parameter was dropped as soon as the user bounced through /log-in on the way to /start. Without it, the account-creation request had no record of where the user came from, so these sign-ups couldn't be attributed to Akismet.

The change is deliberately generic: any product that sends users to log-in with a ref now keeps it through to account creation, rather than adding an Akismet-specific path.

Testing Instructions

1. Visit /log-in?ref=akismet and click Create an account.
2. Confirm the URL you land on is /start?ref=akismet (or /start/account?redirect_to=…&ref=akismet when a redirect is present).
3. Complete sign-up and confirm the POST to /users/new (or /users/social/new for social sign-up) includes ref=akismet in the payload.
4. Repeat with /log-in?ref= and with no ref at all — the sign-up URL and payload should be unchanged from trunk.
5. yarn test-client client/lib/login/test/index.js

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? 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]

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[github-actions]

Looks like one of the E2E tests has failed.

You can fix them following these steps:

1. Check out this branch locally:

   gh pr checkout 113577

2. Start Claude Code in the repo:

   claude

3. Run the /fix-e2e-tests skill, passing this PR number:

   /fix-e2e-tests 113577