Fix Dialog background scroll

[liamsms]

Closes #3906

• Sets the root overflow to 'hidden' to prevent background scrolling while the Dialog is open, same approach as MUI.
• Scrolling within the Dialog still works as expected.
• Moved the cleanup callback to a new useEffect with no dependencies to prevent a race condition where the overflow style was sometimes not removed.
• Tested on Chrome/Windows.

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Latest commit: e2e20a3

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[liamsms]

May require additional testing on a Mac due to potential differences in scroll behavior, eg. shifts in positioning when the scrollbar appears/disappears.