[Bugfix] Restore Gradio demo dependency (#5632)

Signed-off-by: maithilijoshi20 <maithilijoshi2003@gmail.com>
Co-authored-by: maithilijoshi20 <maithilijoshi2003@gmail.com>

pyproject.toml

@@ -75,6 +75,7 @@ dev = [
 ]
 
 demo = [
+    "gradio>=6.7.0",
     "opencv-python>=4.12.0.88",  # cli/stream_client frame reading
     "requests>=2.28.0",  # cli/stream_client HTTP
     "cn2an>=0.5.22",  # Chinese number normalization for TTS (e.g. "1次" -> "一次")