Enhance section movement validation in TrainingUnitSectionsEditor

- Added a new check to prevent section movement across slots that crosses the boundary between 'parallel' and 'whole_group' phases, improving the logic for section management and ensuring valid operations during edits.

frontend/src/components/TrainingUnitSectionsEditor.jsx

@@ -928,9 +928,23 @@ export default function TrainingUnitSectionsEditor({
       }
     }
 
+    let crossesParallelWholePlanBoundary = false
     if (
       enableParallelPhaseControls &&
-      (insertBeforeIdx === fromSi || insertBeforeIdx === fromSi + 1)
+      insertBeforeIdx >= 0 &&
+      insertBeforeIdx < list.length
+    ) {
+      const fromK = list[fromSi]?.planLoc?.phaseKind
+      const toK = list[insertBeforeIdx]?.planLoc?.phaseKind
+      crossesParallelWholePlanBoundary =
+        (fromK === 'parallel' && toK === 'whole_group') ||
+        (fromK === 'whole_group' && toK === 'parallel')
+    }
+
+    if (
+      enableParallelPhaseControls &&
+      (insertBeforeIdx === fromSi || insertBeforeIdx === fromSi + 1) &&
+      !crossesParallelWholePlanBoundary
     ) {
       return
     }