remove create cell button background on hover (#7298)

## 📝 Summary

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Provide a concise summary of what this pull request is addressing.

If this PR fixes any issues, list them here by number (e.g., Fixes
#123).
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When hovering on the cell-button, it would show a light gray background.
This PR removes that.

This was fixed in an earlier PR but was accidentally removed.

gray border:
<img width="337" height="171" alt="image"
src="https://github.com/user-attachments/assets/a0e72ded-4463-4fea-8c63-cd17cf4f6fb1"
/>

## 🔍 Description of Changes

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## 📋 Checklist

- [x] I have read the [contributor
guidelines](https://github.com/marimo-team/marimo/blob/main/CONTRIBUTING.md).
- [ ] For large changes, or changes that affect the public API: this
change was discussed or approved through an issue, on
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[discussions](https://github.com/marimo-team/marimo/discussions) (Please
provide a link if applicable).
- [ ] I have added tests for the changes made.
- [x] I have run the code and verified that it works as expected.

Author: Light2Dark

frontend/src/components/editor/cell/CreateCellButton.tsx

@@ -113,7 +113,7 @@ export const CreateCellButton = ({
       <DropdownMenuTrigger asChild={true} onPointerDown={handleButtonClick}>
         <Button
           className={cn(
-            "border-none hover-action shadow-none! bg-transparent focus-visible:outline-none",
+            "border-none hover-action shadow-none! bg-transparent! focus-visible:outline-none",
             isAppInteractionDisabled(connectionState) && " inactive-button",
           )}
           onMouseDown={Events.preventFocus}