fix: manual node setup 'Node ID Required' error

Fixed issue where discovered nodes and manual node setup would incorrectly
call updateNode instead of addNode. The problem was that discovered nodes
were set with an empty string ID, which made the code think it was editing
an existing node.

Now properly checks for both editingNode existence AND a valid ID before
deciding whether to update or add.

Fixes #289

frontend-modern/src/components/Settings/Settings.tsx

@@ -1727,8 +1727,8 @@ docker run -d \
         editingNode={editingNode() ?? undefined}
         onSave={async (nodeData) => {
           try {
-            if (editingNode()) {
-              // Update existing node
+            if (editingNode() && editingNode()!.id) {
+              // Update existing node (only if it has a valid ID)
               await NodesAPI.updateNode(editingNode()!.id, nodeData as NodeConfig);
               
               // Update local state