Add class=error to error boxes

Author: danr

cellpainter/cellpainter/gui/main.py

@@ -350,6 +350,7 @@ def world_key(d: dict[str, str]):
         )
         if ar.has_error():
             box = div(
+                _class='error',
                 border='2px var(--red) solid',
                 px=8,
                 py=4,