I tried to use iFrame cards rather than the sidebar to display apps / addons - e.g. AdGuard Home.
This works - but then the sidebar is displayed as well next to the addon ui.
So I get the sidebar twice displayed - this is not nice.
Go to the configuration of the AdGuard app, and set a web interface port in the Network settings. (Note: this likely exposes it to your whole .178 LAN).
Then,point it the iframe at http://192.168.178.14:<PORTNUMBER>
That in theory should work, as long as AdGuard doesn't use CSP/X-Frame Options. If it does, your iframe will never load.
That is what I have done. This works for Adguard but not every app / addon has the option to define a separate port. My sidebar is getting to long .....
I need to think about what kind of apps / addons I shall integrate in HA or to be installed outside HA.
I am using Proxmox.