docs: use CSS frame with centered demo gif

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: YuweiXiao

README.md

@@ -16,9 +16,9 @@ PostgreSQL extension for real-time CDC to pg_ducklake
 
 ![pg_duckpipe architecture](images/arch.png)
 
-<table><tr><td>
-<img src="images/demo.gif" alt="pg_duckpipe demo" width="800">
-</td></tr></table>
+<div align="center">
+<img src="images/demo.gif" alt="pg_duckpipe demo" width="80%" style="border: 1px solid #e1e4e8; border-radius: 8px; box-shadow: 0 2px 8px rgba(0,0,0,0.12);">
+</div>
 
 ## Key Features