Use qt-niu display_warning for atlas download/update confirmation

[Ankithg21]

Description

This PR addresses Issue #154 by replacing custom atlas download/update confirmation dialogs with the display_warning dialog from qt-niu, as recommended.

Previously, confirmation dialogs were implemented manually, which resulted in inconsistent UI behavior compared to other BrainGlobe tools.

This update standardizes the dialog handling by using the qt-niu utility, ensuring consistency across the BrainGlobe ecosystem.

References

Closes #154

How has this PR been tested?

• Tested locally after implementing the changes
• Verified that atlas download and update confirmation dialogs appear correctly
• Confirmed that user interaction (accept/cancel) works as expected
• Ensured no regression in atlas loading functionality

Is this a breaking change?

No. This PR does not introduce any breaking changes.

Does this PR require an update to the documentation?

No documentation update is required, as this change improves internal UI consistency without altering user-facing functionality.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit

[adamltyson]

This is on PyPI, it doesn't need to be installed from GitHub

[Ankithg21]

Thanks for pointing that out! I've updated the dependency to install qt-niu from PyPI instead.