Verbum Comments: surface login requirement when in-frame social login is unavailable

[arcangelini]

Proposed changes

On Atomic sites that require login to comment (comment_registration enabled), logged-out visitors were shown a comment form with no way to log in — the social login options don't render in the cross-origin iframe, and submitting the guest form just returns "Sorry, you must be logged in to comment." A dead end.

This surfaces the login requirement up front instead of after a failed submit:

• When login is required but the in-frame login options can't be rendered (cookies blocked), Verbum now shows the "Log in to leave a comment" prompt with a Log in button instead of a guest form that will be rejected.
• The button links to the site's own login (wp-login.php, opened top-level), so the visitor returns authenticated via Jetpack SSO and can comment.

This is a frontend-only change (plus one translatable string) and does not alter behavior for any case other than that specific dead-end state.

Related product discussion/links

• n/a

Does this pull request change what data or activity we track or use?

No.

Testing instructions

1. On an Atomic (WoA) site, enable Settings → Discussion → "Users must be registered and logged in to comment".
2. Open a post as a logged-out visitor.
3. Before: the comment area shows a name/email form; submitting returns "Sorry, you must be logged in to comment" with no login option.
4. After: the comment area shows the "Log in to leave a comment" prompt and a Log in button that takes you to the site login, and no dead-end guest form.
5. Confirm normal (non-registration-required) posts, and Simple sites, are unchanged.

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (WordPress.com Site Helper), and enable the cm-841/surface-login-requirement branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack-mu-wpcom-plugin cm-841/surface-login-requirement

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 2 files.

File	Coverage	Δ%	Δ Uncovered
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/src/components/logged-out.tsx	0/206 (0.00%)	0.00%	51 💔
projects/packages/jetpack-mu-wpcom/src/features/verbum-comments/class-verbum-comments.php	0/378 (0.00%)	0.00%	1 ❤️‍🩹

Full summary · PHP report · JS report

_{If appropriate, add one of these labels to override the failing coverage check: Covered by non-unit tests Use to ignore the Code coverage requirement check when E2Es or other non-unit tests cover the code Coverage tests to be added later Use to ignore the Code coverage requirement check when tests will be added in a follow-up PR I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage.}