Today was not a great day...

flashing a zip via the Web IDE? How exactly did you do this

I downloaded a ZIP to my PC, then uploaded it into the browser. I got it to work using F12 by setting a breakpoint in the stepDownload function and manually setting data.binary_url = '' in the console. (it was undefined)

I imagine the Layout error you hit was because of the bootloader.

Yeah, it was late and I was tired, so it was probably just me forgetting to upload the bootloader.

messing with Storage while out-of-memory probably somehow managed to erase all storage, no idea if it's reproducible

from what you describe it may be even this case https://github.com/espruino/Espruino/iss­ues/2075
otherwise there is issue https://github.com/espruino/Espruino/iss­ues/2009

I doubt it was the watchdog, as the watch didn't reboot by itself.
Basically the watch was showing a "Memory" error, I tried long-pressing BTN3 which gave some error I forgot, and holding BTN1+2 got me stuck at the boot screen.
As for the memory error: I was testing an app which logs all incoming Gadgetbridge messages, and strongly suspect that caused it: it reads the full log into memory every time a new message arrives, and I got a few large ones that day.