Feature request - incoming transaction notifications

It would be amazing to have support for desktop notifications of incoming transactions, so that the user is informed when there's new incoming bitcoin without needing to refresh the browser, or indeed even look at the web interface - this is pretty much the only feature preventing me to fully switch from Electrum at this point.

Taking a cursory look this could be achieved by using ZMQ subscriptions, however as it's not plug-and-play it would probably have to be an advanced setting, as Bitcoin Core default config doesn't enable it (though it's compiled into the official binaries afaik).

The other challenges I can think of, off the top of my head:

- Having cross-platform desktop notification (perhaps just browser notifications instead of OS native would suffice?)
- Extra configuration steps if Specter is connecting to the node via Tor, seeing as the ZMQ port would also need to be published as a separate hidden service
- There's probably a client / frontend websocket component to develop?

Thanks for the great work, Specter is awesome.

No files, tests, or entry points are named. Start by mapping how incoming transactions reach the web interface and how the desktop GUI handles notifications, then investigate the proposed ZMQ subscription and browser or OS notification paths. Done means cross-platform notifications work without refreshing, including the stated Tor configuration case.

Written by the indexing model from the issue text.