Viewport: Fix editing when default viewport is set

[shilman]

Closes #26334

What I did

Fixed a reactivity bug that caused the viewport to snap back to the default viewport when the user tries to edit.

Checklist for Contributors

Testing

1. Added a test for this case
2. Checked that it fails with the old code
3. Checked that it passes with the new

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

See E2E test

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

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