feat(rembg): switch background removal to bria-rmbg model

Replace default u2net with bria-rmbg-2.0 for better quality.
BackgroundRemover now accepts model_name param (defaults to 'bria-rmbg').

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

hy3dshape/hy3dshape/rembg.py

@@ -17,8 +17,8 @@ from rembg import remove, new_session
 
 
 class BackgroundRemover():
-    def __init__(self):
-        self.session = new_session()
+    def __init__(self, model_name: str = "bria-rmbg"):
+        self.session = new_session(model_name)
 
     def __call__(self, image: Image.Image):
         output = remove(image, session=self.session, bgcolor=[255, 255, 255, 0])