Downgrade Gradio to 5.12.0 to fix ZeroGPU crash

Gradio 6.x enables SSR mode by default which is incompatible with
ZeroGPU containers, causing the app to exit immediately with code 0.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -4,7 +4,7 @@ emoji: 📉
 colorFrom: pink
 colorTo: purple
 sdk: gradio
-sdk_version: 6.5.1
+sdk_version: 5.12.0
 app_file: app.py
 pinned: false
 license: apache-2.0