[Self-hosted] Hide Explore button when FLUXER_DISCOVERY_ENABLED is disabled

Currently on self-hosted instances, the Explore button on the sidebar stays when FLUXER_DISCOVERY_ENABLED is set to false. If a user clicks it, it will error with Couldn't load communities Screenshot_20261017_015408.png but the console shows the correct response according to the docs. Screenshot_20261017_015505-1.png I think this could be improved as users will see this and think there's a problem.

Instead of erroring, show the correct response to the user that Discovery is disabled on this instance. Since Explore also shows Apps and Themes as coming soon, I think having the option for operators to choose whether to hide Explore as a whole or hide just Communities would be nice.