Add immediate UI update for Generate and Stop buttons (#18)

### Summary
The Stop button previously appeared only after the backend generation job actually started, leaving users with no cancel affordance while a request sat in the queue. This PR makes the Stop button visible immediately after clicking **Generate Podcast**.

### Change
Adds a single non-queued `.then` step in the click event chain
that hides the Generate button and shows the Stop button instantly, before the queued job begins processing.

demo/gradio_demo.py

@@ -992,6 +992,11 @@ Or paste text directly and it will auto-assign speakers.""",
             inputs=[],
             outputs=[audio_output, complete_audio_output],
             queue=False
+        ).then(  # Immediate UI update to hide Generate, show Stop (non-queued)
+            fn=lambda: (gr.update(visible=False), gr.update(visible=True)),
+            inputs=[],
+            outputs=[generate_btn, stop_btn],
+            queue=False
         ).then(
             fn=generate_podcast_wrapper,
             inputs=[num_speakers, script_input] + speaker_selections + [cfg_scale],