fix: improve workflow validation error logging and display

[elijahr]

Summary

This PR fixes validation error debugging for workflow templates by improving error logging and display.

The main focus is on making workflow validation errors easier to debug by providing clear, actionable error information to developers (via console.error()) while maintaining a good user experience in the toast.

Previously the toast would output incomplete error messaging that was not very useful for debugging.

Now the toast shows the invalid field names in addition to the failure, while maintaining the truncation ("+40 more...") if there are many failures. But the developer console shows the full error object which can be used for debugging.

Also adds support for an optional INVOKEAI_API_BASE_URL env var for development, to allow the frontend server and the InvokeAI API server it proxies to be on separate machines.

Checklist

• [x] The PR has a short but descriptive title, suitable for a changelog
• [n/a] Tests added / updated (if applicable)
• [n/a] ❗Changes to a redux slice have a corresponding migration
• [n/a] Documentation added / updated (if applicable)
• [n/a] Updated What's New copy (if doing a release after this PR)