Automatic refresh of nbdev_preview lost when using output-file on the frontmatter

Using `output-file` on the notebook frontmatter works great to specify a fixed URL. However, I lost the automatic refresh of the output of `nbdev_preview`.

Without `output-file`, any notebook changes saved get immediately reflected on `localhost:3000` when using `nbdev_preview`. This feature is lost when using `output-file`. Is this expected?

Start at the nbdev_preview entry point and reproduce the localhost:3000 refresh behavior with and without output-file in notebook frontmatter. Trace how the fixed URL is handled, then determine whether live refresh should be preserved; done means the expected behavior is restored or clearly documented and covered by a regression check.

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