When the UI font size changes, the settings window's scrollbar would
break because list item measurements were cached and not invalidated.

We're using `ListState.measure_all()` to pre-measure all items for an
accurate scrollbar. When font size changes, these cached measurements
become stale but weren't being refreshed.

This commit fixes the issue by calling `ListState.measure_all()` when
settings change, ensuring items are re-measured on the next layout
without affecting scroll position.

Closes #43683

Release Notes:

- Fixed bug where changing the UI Font Size in the Settings UI would
break the scrollbar for the "Appearance" page

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Co-authored-by: Zed Zippy <234243425+zed-zippy[bot]@users.noreply.github.com>