update banner image

[minpeter]

Summary by cubic

Replaced the README banner with a new PNG for sharper rendering and consistent format. Removed assets/banner.jpg and updated the README to use assets/banner.png.

^{Written for commit 865408d. Summary will update on new commits.}

Review in cubic

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[gemini-code-assist]

Code Review

This pull request updates the README.md file to change the file extension of the banner image from .jpg to .png. There are no review comments, and I have no additional feedback to provide.