JITM: center .jitm-card with margin 16px auto (9px auto on mobile)

[enejb]

Proposed changes

Updates the .jitm-card margin so the card is horizontally centered:

• Base: margin: 16px auto; (was 48px 20px 0 auto)
• Mobile (<480px): margin: 9px auto;

Values use the existing rem.convert() helper and the file's <480px breakpoint convention.

Related product discussion/links

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

No.

Testing instructions

• Open any Jetpack/Woo admin screen (or a screen where a JITM renders) with a JITM visible.
• Confirm the .jitm-card is horizontally centered with a 16px top/bottom margin.
• Narrow the viewport below 480px and confirm the vertical margin tightens to 9px, still centered.

Screenshots

Captured on a live Jurassic Ninja site on the Plugins screen (a generic admin page where the base .jitm-card rule applies with no per-context override). A test JITM was injected via filter. Computed margin: before 48px 20px 0 0 → after 16px auto (desktop), 9px auto (<480px).

Desktop (1440px)

Before	After

Mobile (390px)

Before	After

[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 or WordPress.com Site Helper), and enable the update/jitm-card-margins branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack update/jitm-card-margins

bin/jetpack-downloader test jetpack-mu-wpcom-plugin update/jitm-card-margins

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• 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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Code Coverage Summary

This PR did not change code coverage!

That could be good or bad, depending on the situation. Everything covered before, and still is? Great! Nothing was covered before? Not so great. 🤷

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