fix: make annotateDirtyNodesWithError return new nodes instead of mutating

node.data.error and node.data.extendedError were being set directly on
node objects, bypassing the immutable update pattern used elsewhere in
the codebase. Both call sites (payload-limit path and network-error
path) now consume the returned mapped array.

Author: Av1ralS1ngh

src/lib/graphUtils.ts

@@ -106,12 +106,10 @@ async function loadBackendLimits(baseUrl: string): Promise<BackendLimits> {
 function annotateDirtyNodesWithError(
   nodes: Node<CalculationNodeData>[],
   message: string
-) {
-  nodes.forEach((node) => {
-    if (node.data?.dirty) {
-      node.data.error = true;
-      node.data.extendedError = message;
-    }
+): Node<CalculationNodeData>[] {
+  return nodes.map((node) => {
+    if (!node.data?.dirty) return node;
+    return { ...node, data: { ...node.data, error: true, extendedError: message } };
   });
 }
 
@@ -178,9 +176,8 @@ export async function recalculateGraph(
     const limitMessage = `Request is ${formatBytes(
       payloadBytes
     )}, over the server limit (${formatBytes(maxPayloadBytes)}).`;
-    annotateDirtyNodesWithError(nodes, limitMessage);
     return {
-      nodes,
+      nodes: annotateDirtyNodesWithError(nodes, limitMessage),
       version,
       errors: [{ nodeId: PAYLOAD_LIMIT_NODE_ID, error: limitMessage }],
     };
@@ -223,11 +220,8 @@ export async function recalculateGraph(
       ? "Backend not running. Start it with: python routes.py"
       : "Cannot connect to calculation service. Please try again later.";
 
-    // Mark all dirty nodes with error
-    annotateDirtyNodesWithError(nodes, message);
-
     return {
-      nodes: nodes,
+      nodes: annotateDirtyNodesWithError(nodes, message),
       version: version,
       errors: [], // No system error - the node errors are enough
     };