Pin gradio to 6.20.0 (sdk_version + requirements.txt)

Match the gr.Workflow frontend (mrfakename PR #92 / akhaliq/z-image-workflow
test mirror), which is pinned to 6.20.0. Keeping the backend on the same
gradio version avoids a cross-version gradio_client/call_space mismatch
when the canvas forwards the visitor's HF OAuth token to /generate_image.

README.md

@@ -4,7 +4,7 @@ emoji: 🖼️
 colorFrom: yellow
 colorTo: yellow
 sdk: gradio
-sdk_version: 6.0.1
+sdk_version: 6.20.0
 app_file: app.py
 pinned: true
 ---

requirements.txt

@@ -1,4 +1,4 @@
-gradio
+gradio==6.20.0
 git+https://github.com/huggingface/diffusers
 transformers
 kernels