docs: update README with demo video

Replaced demo media placeholder with a video element.

Author: maitrungduc1410

README.md

@@ -22,11 +22,9 @@ and real CPython. No server does the work.
 
 </div>
 
-<!-- DEMO MEDIA goes here: a GIF via ![alt](url), or a bare GitHub-hosted video URL
-     on its own line (.mp4/.mov/.webm, H.264). Limits: 10 MB for images/GIFs, and
-     10 MB for video on a free plan / 100 MB on a paid one. Upload by dragging the
-     file into a GitHub issue comment and copying the generated CDN link. Do not
-     commit the binary, or every clone pays for it forever. -->
+<div align="center">
+  <video src="https://github.com/user-attachments/assets/5ee626a8-4970-4c1e-af77-8e2f4c5b7da4" controls loop muted></video>
+</div>
 
 **[Open the Studio](https://vivari.run/studio/)** to try it without installing
 anything: pick a template, run `npm install && npm run dev`, and watch a real dev
@@ -197,4 +195,4 @@ Releases are cut from the manual **Publish SDK** GitHub Actions workflow
 [MIT](LICENSE). Free for any use, commercial or otherwise. There is no
 commercial license and no usage fee: embed it, fork it, ship it.
 
-> **Vivari** *(vih-VAH-ree)* takes its name from the Latin *vivarium*, a self-contained enclosure for living things. That's exactly what it is for a running Node app: a sealed, self-contained environment in the browser where a whole project lives and runs.
+> **Vivari** *(vih-VAH-ree)* takes its name from the Latin *vivarium*, a self-contained enclosure for living things. That's exactly what it is for a running Node app: a sealed, self-contained environment in the browser where a whole project lives and runs.