docs: add demo GIF + MP4 to repo for mobile-friendly README

Author: rz1989s

README.md

@@ -34,7 +34,9 @@ hidden amounts, and compliance viewing keys across 17 chains.**
 
 <div align="center">
 
-https://github.com/user-attachments/assets/a4a87b0c-7168-4677-b8e6-457debb98cb0
+![Sipher Demo](assets/demo.gif)
+
+*[Watch in full quality (MP4)](assets/demo.mp4)*
 
 </div>