fix: hide built-in Gradio video download button on output slots

The Gradio gr.Video download button always points to the originally generated
file and doesn't update after regen. Replaced by the Video download link in
the waveform panel which always reflects the latest mux.

app.py

@@ -1445,7 +1445,8 @@ def _make_output_slots(tab_prefix: str) -> tuple:
         slot_id = f"{tab_prefix}_{i}"
         with gr.Group(visible=(i == 0)) as g:
             vids.append(gr.Video(label=f"Generation {i+1} — Video",
-                                 elem_id=f"slot_vid_{slot_id}"))
+                                 elem_id=f"slot_vid_{slot_id}",
+                                 show_download_button=False))
             waveforms.append(gr.HTML(
                 value="<p style='color:#888;font-size:12px'>Generate audio to see waveform.</p>",
                 elem_id=f"slot_wave_{slot_id}",