Use `demo` as Gradio entrypoint identifier

- Rename Gradio Blocks context in `src/app.py` from `iface` to `demo` to match official docs
- Update root-level `app.py` wrapper to import and launch `demo` instead of `iface`

app.py

@@ -1,5 +1,5 @@
 # app.py at repo root
-from src.app import iface
+from src.app import demo
 
 if __name__ == "__main__":
-    iface.launch()
+    demo.launch()