Warn that .local mDNS doesn't work over flatpak

[sm-Fifteen]

As of right now, if I install butler through Flathub and try connecting to http://homeassistant.local:8123 (as I would through my web browser), I get a white page with the following error:

Error resolving “homeassistant.local”: Temporary failure in name resolution

This is a shortcoming of FlatPak at the moment, which has no mean of performing mDNS resolution unless the network is configured in a specific way to work around this, and is currently waiting for a portal to be added to resolve that.

I happen to be familiar enough with flatpak and mDNS to figure this out, but there should probably be a warning to that effect in the interface itself.