Ensure the vertical scroll bar on setup page is flush with graphic

[richard-cox]

Summary

Fixes: #12145

Occurred changes and/or fixed issues

• See screenshots
  • scroll bar flush with graphic
  • give more room for form
• Held off doing this as there was a PR that totally changed the whole setup / login page flow, however i think that's now DOA
• So this is a quick fix for a long time annoyance

Technical notes summary

• The file is a mess of styles, will need tidying up one day

Areas or cases that should be tested

• setup pages
• loign pages (just in case)

Screenshot/Video

Before

After

Checklist

• The PR is linked to an issue and the linked issue has a Milestone, or no issue is needed
• The PR has a Milestone
• The PR template has been filled out
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• The PR has automated tests or clear instructions for manual tests and the linked issue has appropriate QA labels, or tests are not needed
• The PR has reviewed with UX and tested in light and dark mode, or there are no UX changes