deps: migrate the UI to Gradio 6

Gradio was unpinned, so a fresh install pulled Gradio 6, where
gr.Audio no longer accepts show_download_button — the UI crashed on
startup with TypeError. Move forward to Gradio 6 instead of pinning
back to 5:

- requirements: gradio>=6.0.0
- drop the removed show_download_button kwarg (4 call sites)
- requirements: transformers>=4.57.0 — Gradio 6 requires
  huggingface-hub>=1.0, which transformers==4.50.0 forbids. Only
  UMT5EncoderModel and AutoTokenizer are used, so the bump is safe.

Verified: pip check clean, UI renders and server callbacks work on
gradio 6.26.0 / transformers 5.16.1 / huggingface-hub 1.30.0.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

acestep/ui/components.py

@@ -134,7 +134,7 @@ def create_text2music_ui(
                 )
                 lora_weight = gr.Number(value=1.0, label="Lora weight", step=0.1, maximum=3, minimum=-3)
 
-            ref_audio_input = gr.Audio(type="filepath", label="Reference Audio (for Audio2Audio)", visible=False, elem_id="ref_audio_input", show_download_button=True)
+            ref_audio_input = gr.Audio(type="filepath", label="Reference Audio (for Audio2Audio)", visible=False, elem_id="ref_audio_input")
             ref_audio_strength = gr.Slider(
                 label="Refer audio strength",
                 minimum=0.0,
@@ -403,7 +403,6 @@ def create_text2music_ui(
                     type="filepath",
                     visible=False,
                     elem_id="repaint_source_audio_upload",
-                    show_download_button=True,
                 )
                 repaint_source.change(
                     fn=lambda x: gr.update(
@@ -572,7 +571,6 @@ def create_text2music_ui(
                     type="filepath",
                     visible=False,
                     elem_id="edit_source_audio_upload",
-                    show_download_button=True,
                 )
                 edit_source.change(
                     fn=lambda x: gr.update(
@@ -724,7 +722,6 @@ def create_text2music_ui(
                     type="filepath",
                     visible=False,
                     elem_id="extend_source_audio_upload",
-                    show_download_button=True,
                 )
                 extend_source.change(
                     fn=lambda x: gr.update(