Third-party nodes not loading when Dashboard hosted on proxy

### Current Behavior

Details: https://discourse.nodered.org/t/d2-third-party-nodes-do-not-appear-blocked-due-to-forbidden-mime-text-html/88916

Reported error:

```
/resources/@colinl/node-red-dashboard-2-ui-gauge-classic/ui-gauge-classic.umd.js” zostało zablokowane z powodu niedozwolonego typu MIME („text/html”).`
```

I suspect we have a hardcoded path when loading third-party nodes

### Expected Behavior

Third party widgets should be loading as core nodes do.

### Steps To Reproduce

- Will need to setup an nginx proxy
- Install a third-party widget

### Environment

- Dashboard version:
- Node-RED version:
- Node.js version:
- npm version:
- Platform/OS:
- Browser:

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate