fix: Prune invalid builder mappings on load (#9376)

## Summary

- extract resolveNode to reusable util
- remove mid builder pruning
- handle missing widgets with label

## Review Focus

`resolveNode` was simplified for subgraphs by calling getNodeById on
each of the subgraphs instead of searching their inner nodes manually.

## Screenshots (if applicable)

"Widget not visible"
<img width="657" height="822" alt="image"
src="https://github.com/user-attachments/assets/ab7d1e87-3210-4e54-876a-07881974b5c7"
/>
<img width="674" height="375" alt="image"
src="https://github.com/user-attachments/assets/c50ec871-d423-43d6-8e1e-7b1a362f621c"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9376-fix-Prune-invalid-builder-mappings-on-load-3196d73d3650811280c2d459ed0271af)
by [Unito](https://www.unito.io)

src/locales/en/main.json

@@ -3067,7 +3067,8 @@
       "promptAddOutputs": "Click on output nodes to add them here. These will be the generated results.",
       "noOutputs": "No output nodes added yet",
       "outputsDesc": "Connect at least one output node so users can see results after running.",
-      "outputsExample": "Examples: “Save Image” or “Save Video”"
+      "outputsExample": "Examples: “Save Image” or “Save Video”",
+      "unknownWidget": "Widget not visible"
     },
     "queue": {
       "clickToClear": "Click to clear queue",