graph: session nodes carry no geometry toggle, like utility nodes

The designer's new permanent Session container is a settings node — the
per-node geometry-visibility toggle would be meaningless on it, so
toggle_rect skips the "session" type the same way it skips "utility".

 src/widget/display/graph.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/widget/display/graph.rs b/src/widget/display/graph.rs
index a3c52c2..3b55c99 100644
--- a/src/widget/display/graph.rs
+++ b/src/widget/display/graph.rs
@@ -237,7 +237,9 @@ impl Graph {
 
     pub fn toggle_rect(&self, idx: usize) -> Option<(f32, f32, f32, f32)> {
         if let Some(node) = self.nodes.get(idx) {
-            if node.node_type == "utility" {
+            // Settings containers have no geometry to toggle: utility nodes,
+            // and the designer's session node that now nests them.
+            if node.node_type == "utility" || node.node_type == "session" {
                 return None;
             }
         }