Dashboard: fix launch celebration modal Get domain link 500 in standalone dashboard

[StevenDufresne]

Fixes DOTMSD-1378

Proposed Changes

• Build the "Get your domain" link in the site launch celebration modal via the shared getAddSiteDomainUrl() helper instead of a hardcoded relative /domains/add/<slug> path.
• Strengthen the modal's upsell test to assert the link points at the environment-aware add-domain URL.

Why are these changes being made?

• In the standalone Multi-site Dashboard, a relative /domains/add/<slug> resolves against the current site host (e.g. <slug>.wpcomstaging.com), which has no add-domain flow — so the link 500s.
• The shared helper already handles both environments correctly: an absolute wordpress.com URL when standalone, and a relative path in the Calypso backport. Three other components already use it; this brings the modal in line.

Testing Instructions

• In the multi-site dashboard, open a site's Performance page for a site without a custom domain.
• Launch the site so the celebration modal appears.
• Click "Get your domain" and confirm it lands on the wordpress.com add-domain flow rather than a 500.
• yarn test-client client/dashboard/sites/site-launch-celebration-modal/test/index.test.tsx

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]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-186022

Jetpack Cloud Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-186022&env=jetpack

Automattic for Agencies Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-186022&env=a8c-for-agencies

Dashboard Live (dotcom) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-186022&env=dashboard

Dashboard Live (A4A) (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-186022&env=dashboard-a4a

[paulopmt1]

It's working as expected, thanks for the fix @StevenDufresne!

Tested prod:

With the fix it properly redirects: