v1.7.2 - self-hosted server support

Debug build with the "Connect to a self-hosted server" feature (see README).
No server is hardcoded - defaults to the official stoat.chat instance until
you point it elsewhere from the login screen.

Installs alongside the official app (separate package ID, chat.revolt.debug).
Push notifications don't work in this build (placeholder Firebase config,
see docs/contributing/setup for why) - messaging works fully either way.