[BUG] DASH_PROXY not used by JupyterDash

[benmoss]

Describe your context

• replace the result of pip list | grep dash below (using pixi so this is the conda package version)

dash                           3.2.0           pyhd8ed1ab_0          5.3 MiB    conda  https://conda.anaconda.org/conda-forge/

Describe the bug
When I set DASH_PROXY so that Dash reports a different URL than the listen address it is not respected by Dash's Jupyter support.

Expected behavior

The external URL and iframe that Dash renders should use the DASH_PROXY

https://github.com/benmoss/jupyter-dash-proxy demonstrates the issue, just needs the https://pixi.sh/ package manager to install and run.