use overflow: hidden on splitter panes

This fixes problems with double scroll bars showing up.

Fixes: https://github.com/pybricks/pybricks-code/issues/406
Fixes: https://github.com/pybricks/pybricks-code/issues/407

src/app/app.scss

@@ -22,6 +22,10 @@
     background-color: $pt-app-background-color;
 }
 
+.splitter-layout .layout-pane {
+    overflow: hidden;
+}
+
 .#{$ns}-dark .pb-app-terminal-padding {
     background-color: black;
 }