App development workflow

Yes. But how do you use an app with 3+ files in the web IDE?

You can open 3x Web IDE windows connected to the same watch usually :)

And how do you send the test notification in the web IDE?

You can just paste the GB(...) commands into the left-hand side of the IDE. Some example commands are listed at http://www.espruino.com/Gadgetbridge#mes­sages-sent-to-bangle-js-from-phone

The nice thing about faking it is if there are any errors, they get displayed in the IDE console where you can pick them up :)