Fix height calculation to use zoom-adjusted values

Use (b - t) instead of v->height() for consistency with zoom-scaled bounding box calculations. This ensures correct vertical spacing of edge variables.

Co-authored-by: highperformancecoder <3075825+highperformancecoder@users.noreply.github.com>

Author: Copilot

model/group.cc

@@ -1221,9 +1221,9 @@ namespace minsky
               top = t;
             }
           else if (i % 2)
-            top -= v->height();
+            top -= (b - t);
           else
-            bottom += v->height();
+            bottom += (b - t);
         }
     };