fix: tweak modal iframe size

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

This PR is a small tweak to the default iframe size on load (thanks @thomasguillot!).

See p1734437900696829-slack-newspack-reader-activation.

How to test the changes in this Pull Request:

1. Apply the PR and run npm run build
2. Load the modal checkout.
3. Confirm the iframe's initial size uses --newspack-ui-spacer-9, not --newspack-ui-spacer-8 (the 48px fallback is correct, though). Note: the applied height will be overridden by inline JS.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[thomasguillot]

I'm not a developer, but I approve this PR!

Edit: I can't approve this PR 🤣

[laurelfulford]

Thanks @chickenn00dle and @thomasguillot!

[github-actions]

Hey @laurelfulford, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

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