fix: switch GitHub Stars badge from social to flat style

Social-style shields.io badges intermittently render as "invalid"
due to GitHub API rate limiting. Flat style is more reliable.

Author: saschabuehrle

README.md

@@ -21,7 +21,7 @@
 [![Python Docs](https://img.shields.io/badge/docs-Python-blue)](./docs/)
 [![TypeScript Docs](https://img.shields.io/badge/docs-TypeScript-red)](./docs/)
 [![X Follow](https://img.shields.io/twitter/follow/saschabuehrle?style=social)](https://x.com/saschabuehrle)
-[![GitHub Stars](https://img.shields.io/github/stars/lemony-ai/cascadeflow?style=social)](https://github.com/lemony-ai/cascadeflow)
+[![GitHub Stars](https://img.shields.io/github/stars/lemony-ai/cascadeflow?style=flat&color=yellow&label=Stars)](https://github.com/lemony-ai/cascadeflow/stargazers)
 
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