fix: pin gradio<6.12 — 6.12 breaks iframe loading via bad by-subdomain call

Gradio 6.12 introduced a frontend call to:
GET https://huggingface.co/api/spaces/by-subdomain/{space-id}

For Docker-based spaces, the HF API returns 400 Bad Request:
"Invalid repo name: cronos3k/LARQL-Explorer - repo name includes
an url-encoded slash"

Gradio 6.12 throws an uncaught exception on the 400, crashing the
entire Gradio initialization in iframe context. App is stuck at
"Loading..." indefinitely.

Fix: pin to <6.12 until Gradio fixes the by-subdomain error handling.

requirements.txt

@@ -1,2 +1,2 @@
-gradio>=6.0.0
+gradio>=6.0.0,<6.12
 huggingface_hub>=0.20.0