Replace embedded demo video with badge link

Replace the inline <video> tag with a shield-style clickable badge that links to public/demo_video/demo.mp4 and add a short note prompting users to click to download and watch the demo. This simplifies the README layout and avoids embedding a large video directly.

Author: DongDuong2001

README.md

@@ -13,7 +13,9 @@
 
 ## Demo
 
-<video src="public/demo_video/demo.mp4" controls width="100%"></video>
+[![Demo Video](https://img.shields.io/badge/▶️_Watch-Demo_Video-red?style=for-the-badge)](public/demo_video/demo.mp4)
+
+*Click above to download and watch the demo video*
 
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