Closes #1947

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## Summary by cubic
Fixed the Show Token Usage button so it toggles on a single click.
Switched to a functional state updater to prevent stale state and double
toggles.

- **Bug Fixes**
- Replace setShowTokenBar(!showTokenBar) with a memoized toggle using
setShowTokenBar(prev => !prev) to ensure reliable toggle and immediate
active styling.

<sup>Written for commit e1351b9044a447515d4dc817cdff83fff50a57f4.
Summary will update automatically on new commits.</sup>

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