Restore coloring for NA values

[alexhallam]

Re-enable coloring for NA values by using a padded-aware NA detection.

The previous is_na function did not correctly identify NA values after they had been padded for display alignment, causing them to appear white. Switching to is_na_string_padded restores the intended coloring.

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[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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