fix: ensure renderer is initialized before operations

- Add _ensure_initialized() method to Viewer3DWidget
- Queue pending meshes until widget is shown
- Add remove_mesh() method to PygfxRenderer
- Fix all viewer methods to check initialization

src/fluency/rendering/pygfx_renderer.py

@@ -264,6 +264,13 @@ class PygfxRenderer(Renderer):
             return True
         return False
 
+    def remove_mesh(self, mesh_id: str) -> bool:
+        """Remove a mesh by its ID."""
+        for obj in self._objects:
+            if obj.name == mesh_id:
+                return self.remove_object(obj)
+        return False
+
     def clear_scene(self) -> None:
         """Remove all objects from the scene."""
         for obj in self._objects[:]: