Use absolute iframe URL when available

app.py

@@ -319,7 +319,9 @@ def _build_chemiscope_embed(dataset_payload: dict, dataset_path: str | Path) ->
     viewer_path = dataset_file.parent / viewer_name
     viewer_path.write_text(combined, encoding="utf-8")
 
-    iframe_src = f"/file={urllib.parse.quote(viewer_path.as_posix())}"
+    view_rel_path = f"/file={urllib.parse.quote(viewer_path.as_posix())}"
+    space_origin = _infer_space_origin()
+    iframe_src = f"{space_origin}{view_rel_path}" if space_origin else view_rel_path
     return (
         "<div style='width:100%;'>"
         "<iframe "