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>

requirements.txt

@@ -1,6 +1,6 @@
 datasets==3.4.1
 diffusers>=0.33.0
-gradio
+gradio>=6.0.0
 librosa==0.11.0
 loguru==0.7.3
 matplotlib==3.10.1
@@ -12,7 +12,7 @@ torch
 torchaudio
 torchvision
 tqdm
-transformers==4.50.0
+transformers>=4.57.0
 py3langid==0.3.0
 hangul-romanize==0.1.0
 num2words==0.5.14