fix: Incorrect Z index for sheet header cells within table widget

[GopalVerma1303]

Fixes #3396

This pull request addresses issue #3396 by adjusting the z-index values for sheet components. The changes ensure proper layering, preventing the reported problem of incorrect z-index for sheet header cells within the table widget.

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[GopalVerma1303]

Hey @seancolsen! Please review.
Thanks!

[seancolsen]

I'm closing this because:

1. After Fix active cell displaying above row header cell #3382 was merged, this PR now has enough merge conflicts that it probably makes sense to start over from scratch — both in terms of code changes and just conceptually, given the re-ordering I did to the z-index values in that PR.
2. Further, the changes here don't seem to affect the right part of the app. I've added a comment in the issue which I hope will clarify where the issue is occurring.