So, this is what Google translate made of your story (translated freely, and summarised heavily):
"Core boots up, works for 2 seconds, then goes to fixed Magenta, and stays that way. According to a forum post, I should update my firmware over USB in DFU mode. I have installed the CLI as documented over here. So far, I've only worked with the Web IDE, and it seems that DFU-UTIL cannot find my Core. I then followed this tutorial. The attached image is what I currently have in my console. What do I do, what have I forgotten? My Core is connected in DFU mode (yellow LED flashing, and it appears with the right driver in peripheral)"
Have you got a screenshot of how your device looks in the device manager? Mine looks like this when it's in DFU mode:
Also, have you tried a factory reset? That might help as well.
I'd highly recommend posting in English in the future, since that's the language that's understood by most, if not all, people on the forum here. Posting in French will severely limit the amount of people able to help you. I have thrown it through Google translate to make some sense out of it, since my high school French was a bit too long ago. Imagining that not everyone is going to want to do that, you're losing a lot of people who are potentially able to help you.