docs: update README with screencast demo

- Replace static screenshot with a dynamic screencast GIF
- Add `short-demo.gif` to `client/public/screenshots`

This change provides a more engaging and informative demonstration of the terminal portfolio's functionality, enhancing the user experience by showcasing the interactive features in action.

Author: subhayu99

README.md

@@ -4,7 +4,7 @@
 
 **Live Demo:** [subhayu99.github.io](https://subhayu99.github.io)
 
-![Terminal Portfolio Screenshot](https://via.placeholder.com/800x400/000000/00FF00?text=Terminal+Portfolio+Demo)
+![Terminal Portfolio Screencast](client/public/screenshots/short-demo.gif)
 
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