Construct 2 can export your project as a traditional desktop app with NW.Js , which is basically a standalone version of the Google Chrome browser. In other wor...
how do we disable the F12 dev mode ???
Just make a "on F12 pressed" event that does something else but that's far from foolproof since for example it'll still work if you have anything selected such as a textbox or button object(and even if it did you can still use right click and inspect on those things). But that's probably still good enough to prevent someone accidentally figuring out that they can do that.
Why do you want to?
People on steam can get into the files
They can still do that without the F12 dev mode. For full protection you need DRM, which is a big topic and still can never completely protect your game.