Works now also with HTTPS, remote access, and reverse proxy.
Use img instead of iframe:
Allows to set max-width instead of width/height preserving aspect ratio.
Avoids infinite loading progress bar.
Avoids deprecated iframe attributes.
Place fieldset and preview panel into same row.
Make URL readonly input and copy to clipboard on click.
Add snapshot URL.
Alert when preview not possible and hint at opening manually since Chromium
blocks subresource URLs with embedded credentials as first step to stop
supporting these deprecated URLs.
imginstead ofiframe:max-widthinstead ofwidth/heightpreserving aspect ratio.iframeattributes.