Subscribe Block: enable customizing the button-only pop-up via the editor

[gabrielcaires]

Fixes #

Proposed changes

• Add Jetpack_Subscribe_Modal_Button, a local pop-up shown when a visitor clicks a "Button only" style Subscribe block, mirroring Jetpack_Subscribe_Modal's existing pattern (editable Site Editor template part, no separate settings field).
• Update the Subscribe block's frontend script: when a "Button only" style form is submitted with no email, open the new local pop-up instead of the checkout iframe. Submitting the pop-up's own embedded Subscribe block hands off to the checkout iframe as before, this time with the email already populated — skipping the iframe's email-collection screen, same as every other Subscribe block style already does.
• Replace the "Subscribe modal heading" settings card (a free-text field) with a 5th entry in the existing "Subscriptions" placements grid — "Subscribe button pop-up," always-on, with the same "Preview and edit" link into the Site Editor as its siblings.
• Seed the new template part's default heading from the legacy subscribe_modal_heading site option, so a site that already customized it doesn't lose that text.

Related product discussion/links

• Linear: NL-783, NL-25

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

No.

Testing instructions

• Add a Subscribe block set to the "Button only" style to a post accessible to everybody.
• View the post logged out.
• Click the Subscribe button — a local pop-up should open (no iframe yet), with a heading and an email field.
• Fill in an email and submit — the checkout iframe (subscribe.wordpress.com/memberships) should now open with the email already populated, skipping straight to plan selection/thanks (not asking for email again).
• In Newsletter Settings, confirm the "Subscribe modal heading" text field is gone, and the "Subscriptions" grid now shows a 5th "Subscribe button pop-up" card with a "Preview and edit" link that opens the jetpack-subscribe-modal-button template part in the Site Editor.

🤖 Generated with Claude Code

[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 (Jetpack), and enable the add/subscribe-button-only-local-popup branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack add/subscribe-button-only-local-popup

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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Jetpack plugin:

The Jetpack plugin has different release cadences depending on the platform:

• WordPress.com Simple releases happen as soon as you deploy your changes after merging this PR (PCYsg-Jjm-p2).
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[jp-launch-control]

Code Coverage Summary

Cannot generate coverage summary while tests are failing. 🤐

Please fix the tests, or re-run the Code coverage job if it was something being flaky.

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