Make _get_device() ZeroGPU-aware so models target CUDA

On ZeroGPU Spaces torch.cuda.is_available() returns False at
module-load time because the GPU is only attached inside @spaces.GPU.
Check spaces.config.Config.zero_gpu as a fallback so all models are
placed on CUDA and work correctly once the GPU is attached.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

webui.py

@@ -140,8 +140,21 @@ def _clear_target_meta_unless_example(_audio, skip_count):
 
 
 def _get_device() -> str:
-    """Use CUDA if available, else CPU (e.g. for CI or CPU-only environments)."""
-    return "cuda:0" if torch.cuda.is_available() else "cpu"
+    """Use CUDA if available, else CPU (e.g. for CI or CPU-only environments).
+
+    On ZeroGPU Spaces the real GPU is only attached inside @spaces.GPU, so
+    torch.cuda.is_available() returns False at module-load time.  We still
+    want models to target CUDA so they run on the GPU once it is attached.
+    """
+    if torch.cuda.is_available():
+        return "cuda:0"
+    try:
+        from spaces.config import Config
+        if Config.zero_gpu:
+            return "cuda:0"
+    except (ImportError, AttributeError):
+        pass
+    return "cpu"
 
 
 def _session_dir_from_target(target_audio_path: str) -> Path: