docs(readme): scale demo reel to 75% width, add dark grey frame (#442)

- Resize embed: width="100%" → width="75%".
- Re-encode WebP with a 4px #3a3a3a border baked in (GitHub's README
  sanitizer doesn't reliably honor inline CSS borders on <img>, so the
  frame is part of the asset).

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -16,7 +16,7 @@
 ---
 
 <div align="center">
-  <img alt="OpenJarvis demo reel" src="assets/openjarvis_demo_reel.webp" width="100%">
+  <img alt="OpenJarvis demo reel" src="assets/openjarvis_demo_reel.webp" width="75%">
 </div>
 
 ---