fix: keep the schedule's stack selected on prefill and edit (#1496)

Opening the New Schedule modal from a stack's Schedule action, or editing
a stack-targeted task, set the node and the stack, but the node-change
effect then ran and cleared the stack, forcing the user to reselect it.

Move the stack-clear out of that effect into the Node picker's change
handler so it only fires on a user-driven node change. A programmatic node
set from prefill or edit now preserves the stack.

frontend/src/components/ScheduledOperationsView.tsx

@@ -201,12 +201,13 @@ export default function ScheduledOperationsView({ filterNodeId, onClearFilter, p
     return () => { cancelled = true; };
   }, [formAction, formTargetId, formNodeId]);
 
-  // Re-fetch stacks when node changes
+  // Re-fetch stacks when the node changes. Clearing a stale stack selection is
+  // done in the Node picker's onValueChange (a user-driven change), not here, so
+  // a prefilled or edited node keeps its stack instead of being wiped on open.
   useEffect(() => {
     if (!dialogOpen) return;
     if (formNodeId) {
       fetchStacks(formNodeId);
-      setFormTargetId('');
     } else {
       setStacks([]);
     }
@@ -790,7 +791,7 @@ export default function ScheduledOperationsView({ filterNodeId, onClearFilter, p
                   <Combobox
                     options={nodeOptions}
                     value={formNodeId}
-                    onValueChange={setFormNodeId}
+                    onValueChange={(val) => { setFormNodeId(val); setFormTargetId(''); }}
                     placeholder="Select node..."
                   />
                 </div>