This pr fixes#14273.
I have implemented two minor CSS adjustments to resolve the navbar
interaction issue on the sign-in page:
- Removed position: relative and z-index: 1 from signInPageContainer.
- Set z-index: 1 on the SignInPanel div (prevent SignInBackgroundArts
from overlapping the SignInPanel)
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## Summary by CodeRabbit
* **Style**
* Adjusted z-index layering for the sign-in page component.
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