Fix TypeError: pass Path object to ensure_main_model, not string

src/ace_step_engine.py

@@ -77,7 +77,7 @@ class ACEStepEngine:
         try:
             # Use the built-in model downloader
             success, message = ensure_main_model(
-                checkpoints_dir=str(checkpoints_dir),
+                checkpoints_dir=checkpoints_dir,
                 prefer_source="huggingface"  # Use HuggingFace for Spaces
             )