docs: replace the README banner asset

[cyfyifanchen]

Problem

The current README hero banner needs to be replaced. Both README.md and README.zh-CN.md currently reference the same GitHub user-attachment asset as the Raven banner.

Current asset:

• https://github.com/user-attachments/assets/5a99d736-49ee-49c9-8b51-890f14078e78

Scope

• Replace the Raven banner in README.md.
• Replace the matching Raven banner in README.zh-CN.md.
• Keep the English and Chinese READMEs visually aligned.
• Preserve the existing badge and language-link layout below the banner.

Acceptance criteria

• The banner uses the approved new Raven asset.
• The banner renders correctly on GitHub desktop and mobile widths.
• The old banner asset is no longer referenced by either README.
• The README top section still loads without broken images.