fix: prevent SwitchNode from deleting last switch

Add guard in handleDelete and hide the delete button when only one
switch remains, matching ConditionalSwitchNode's existing behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/nodes/SwitchNode.tsx

@@ -92,6 +92,7 @@ export const SwitchNode = memo(({ id, data, selected }: NodeProps<WorkflowNode>)
   // Handle delete switch
   const handleDelete = useCallback(
     (switchId: string) => {
+      if (nodeData.switches.length <= 1) return;
       const updatedSwitches = nodeData.switches.filter((sw) => sw.id !== switchId);
       updateNodeData(id, { switches: updatedSwitches });
 
@@ -227,7 +228,8 @@ export const SwitchNode = memo(({ id, data, selected }: NodeProps<WorkflowNode>)
                   </span>
                 )}
 
-                {/* Delete button */}
+                {/* Delete button (hidden if only one switch) */}
+                {nodeData.switches.length > 1 && (
                   <button
                     className="opacity-0 group-hover:opacity-100 text-neutral-400 hover:text-red-400 transition-opacity"
                     onClick={() => handleDelete(sw.id)}
@@ -247,6 +249,7 @@ export const SwitchNode = memo(({ id, data, selected }: NodeProps<WorkflowNode>)
                       />
                     </svg>
                   </button>
+                )}
               </div>
             ))}