The web IDE doesn't discover the device because the watch has a BANK0 INVALID error and I'm stuck in DfuTarg connect only.
That's fine - it needs to be in DfuTarg mode for the Web IDE firmware update to work. From http://www.espruino.com/Firmware+Update#web-ide :
It is also possible to use the Web IDE to update firmware. Click Settings (top right), then Flasher and follow the instructions.
@Gordon started
Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
That's fine - it needs to be in DfuTarg mode for the Web IDE firmware update to work. From http://www.espruino.com/Firmware+Update#web-ide :